This manual describes how to use gcj, the GNU compiler for the
Java programming language. gcj can generate both .class
files and object files, and it can read both Java source code and
.class files.
Copyright © 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
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The precise terms and conditions for copying, distribution and modification follow.
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does.
Copyright (C) year name of author
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
The hypothetical commands show w and show c should show
the appropriate parts of the General Public License. Of course, the
commands you use may be called something other than show w and
show c; they could even be mouse-clicks or menu items--whatever
suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.
signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
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59 Temple Place, Suite 330, Boston, MA 02111-1307, USA
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As gcj is just another front end to gcc, it supports many
of the same options as gcc. See Option Summary. This manual only documents the
options specific to gcj.
A gcj command is like a gcc command, in that it
consists of a number of options and file names. The following kinds
of input file names are supported:
.java
.class
.zip
.jar
.class files, all of
which are compiled. The archive may be compressed. Files in
an archive which don't end with .class are treated as
resource files; they are copmiled into the resulting object file
as core: URLs.
@file.java source files, but that
may change.)
Each named file is compiled, just as if it had been on the command line.
.a
.so
-llibnamegcc manual.
You can specify more than one input file on the gcj command line,
in which case they will all be compiled. If you specify a
-o FILENAME
option, all the input files will be compiled together, producing a
single output file, named FILENAME.
This is allowed even when using -S or -c,
but not when using -C or --resource.
(This is an extension beyond the what plain gcc allows.)
(If more than one input file is specified, all must currently
be .java files, though we hope to fix this.)
gcj has options to control where it looks to find files it needs.
For instance, gcj might need to load a class that is referenced
by the file it has been asked to compile. Like other compilers for the
Java language, gcj has a notion of a class path. There are
several options and environment variables which can be used to
manipulate the class path. When gcj looks for a given class, it
searches the class path looking for matching .class or
.java file. gcj comes with a built-in class path which
points at the installed libgcj.jar, a file which contains all the
standard classes.
In the below, a directory or path component can refer either to an
actual directory on the filesystem, or to a .zip or .jar
file, which gcj will search as if it is a directory.
-Idir-I are kept in order and prepended
to the class path constructed from all the other options. Unless
compatibility with tools like javac is important, we recommend
always using -I instead of the other options for manipulating the
class path.
--classpath=path--CLASSPATH=path--classpath.
--bootclasspath=pathjava.lang.String.
--extdirs=pathCLASSPATH
The final class path is constructed like so:
-I.
--classpath is specified, its value is appended.
Otherwise, if the CLASSPATH environment variable is specified,
then its value is appended.
Otherwise, the current directory (".") is appended.
--bootclasspath was specified, append its value.
Otherwise, append the built-in system directory, libgcj.jar.
--extdirs was specified, append the contents of the
specified directories at the end of the class path. Otherwise, append
the contents of the built-in extdirs at $(prefix)/share/java/ext.
The classfile built by gcj for the class java.lang.Object
(and placed in libgcj.jar) contains a special zero length
attribute gnu.gcj.gcj-compiled. The compiler looks for this
attribute when loading java.lang.Object and will report an error
if it isn't found, unless it compiles to bytecode (the option
-fforce-classes-archive-check can be used to override this
behavior in this particular case.)
-fforce-classes-archive-check
gnu.gcj.gcj-compiled in java.lang.Object and
issue an error if it isn't found.
The Java programming language uses Unicode throughout. In an effort to
integrate well with other locales, gcj allows .java files
to be written using almost any encoding. gcj knows how to
convert these encodings into its internal encoding at compile time.
You can use the --encoding=NAME option to specify an
encoding (of a particular character set) to use for source files. If
this is not specified, the default encoding comes from your current
locale. If your host system has insufficient locale support, then
gcj assumes the default encoding to be the UTF-8 encoding
of Unicode.
To implement --encoding, gcj simply uses the host
platform's iconv conversion routine. This means that in practice
gcj is limited by the capabilities of the host platform.
The names allowed for the argument --encoding vary from platform
to platform (since they are not standardized anywhere). However,
gcj implements the encoding named UTF-8 internally, so if
you choose to use this for your source files you can be assured that it
will work on every host.
gcj implements several warnings. As with other generic
gcc warnings, if an option of the form -Wfoo enables a
warning, then -Wno-foo will disable it. Here we've chosen to
document the form of the warning which will have an effect - the
default being the opposite of what is listed.
-Wredundant-modifiers
gcj will warn about redundant modifiers. For
instance, it will warn if an interface method is declared public.
-Wextraneous-semicolon
gcj to warn about empty statements. Empty statements
have been deprecated.
-Wno-out-of-date
gcj not to warn when a source file is
newer than its matching class file. By default gcj will warn
about this.
-Wno-deprecated
-Wunused
gcc's -Wunused.
-Wall
-Wredundant-modifiers -Wextraneous-semicolon
-Wunused.
In addition to the many gcc options controlling code generation,
gcj has several options specific to itself.
--main=CLASSNAMEmain method should be invoked when the resulting executable is
run. 1
-Dname[=value]
--main. It defines a system
property named name with value value. If value is not
specified then it defaults to the empty string. These system properties
are initialized at the program's startup and can be retrieved at runtime
using the java.lang.System.getProperty method.
-C
gcj to generate bytecode
(.class files) rather than object code.
--resource resource-namegcj to compile the contents of a
given file to object code so it may be accessed at runtime with the core
protocol handler as core:/resource-nameResourceBundle.getBundle.
The actual file name to be compiled this way must be specified
separately.
-d directory-C, this causes all generated .class files
to be put in the appropriate subdirectory of directory. By
default they will be put in subdirectories of the current working
directory.
-fno-bounds-check
gcj generates code which checks the bounds of all
array indexing operations. With this option, these checks are omitted, which
can improve performance for code that uses arrays extensively. Note that this
can result in unpredictable behavior if the code in question actually does
violate array bounds constraints. It is safe to use this option if you are
sure that your code will never throw an ArrayIndexOutOfBoundsException.
-fno-store-check
ArrayStoreException.
-fjni
gcj there are two options for writing native methods: CNI
and JNI. By default gcj assumes you are using CNI. If you are
compiling a class with native methods, and these methods are implemented
using JNI, then you must use -fjni. This option causes
gcj to generate stubs which will invoke the underlying JNI
methods.
-fno-assert
assert keyword. This is for compatibility
with older versions of the language specification.
-fno-optimize-static-class-initialization
-O2,
gcj will try to optimize the way calls into the runtime are made
to initialize static classes upon their first use (this optimization
isn't carried out if -C was specified.) When compiling to native
code, -fno-optimize-static-class-initialization will turn this
optimization off, regardless of the optimization level in use.
Some gcj code generations options affect the resulting ABI, and
so can only be meaningfully given when libgcj, the runtime
package, is configured. libgcj puts the appropriate options from
this group into a spec file which is read by gcj. These
options are listed here for completeness; if you are using libgcj
then you won't want to touch these options.
-fuse-boehm-gc
gcj to put an object marking descriptor into each
vtable.
-fhash-synchronization
synchronize,
wait, and notify) is pointed to by a word in each object.
With this option gcj assumes that this information is stored in a
hash table and not in the object itself.
-fuse-divide-subroutine
-fcheck-references
As we believe it is important that the Java platform not be fragmented,
gcj and libgcj try to conform to the relevant Java
specifications. However, limited manpower and incomplete and unclear
documentation work against us. So, there are caveats to using
gcj.
This list of compatibility issues is by no means complete.
gcj implements the JDK 1.2 language. It supports inner classes
and the new 1.4 assert keyword. It does not yet support the Java 2
strictfp keyword (it recognizes the keyword but ignores it).
libgcj is largely compatible with the JDK 1.2 libraries.
However, libgcj is missing many packages, most notably
java.awt. There are also individual missing classes and methods.
We currently do not have a list showing differences between
libgcj and the Java 2 platform.
libgcj implementation of a method or class differs
from the JDK implementation. This is not always a bug. Still, if it
affects you, it probably makes sense to report it so that we can discuss
the appropriate response.
gcj does not currently allow for piecemeal replacement of
components within libgcj. Unfortunately, programmers often want
to use newer versions of certain packages, such as those provided by
the Apache Software Foundation's Jakarta project. This has forced us
to place the org.w3c.dom and org.xml.sax packages into
their own libraries, separate from libgcj. If you intend to
use these classes, you must link them explicitly with
-l-org-w3c-dom and -l-org-xml-sax. Future versions of
gcj may not have this restriction.
The main feature of gcj is that it can compile programs written in
the Java programming language to native code. Most extensions that have been
added are to facilitate this functionality.
gcj makes it easy and efficient to mix code written in Java and C++.
See About CNI, for more info on how to use this in your programs.
libgcj system classloader. When trying to load a class
gnu.pkg.SomeClass the system classloader will first try to load the
shared library lib-gnu-pkg-SomeClass.so, if that fails to load the
class then it will try to load lib-gnu-pkg.so and finally when the
class is still not loaded it will try to load lib-gnu.so. Note that
all .s will be transformed into -s and that searching
for inner classes starts with their outermost outer class. If the class
cannot be found this way the system classloader tries to use
the libgcj bytecode interpreter to load the class from the standard
classpath. This process can be controlled to some degree via the
gnu.gcj.runtime.VMClassLoader.library_control property;
See libgcj Runtime Properties.
libgcj includes a special gcjlib URL type. A URL of
this form is like a jar URL, and looks like
gcjlib:/path/to/shared/library.so!/path/to/resource. An access
to one of these URLs causes the shared library to be dlopen()d,
and then the resource is looked for in that library. These URLs are
most useful when used in conjunction with java.net.URLClassLoader.
Note that, due to implementation limitations, currently any such URL
can be accessed by only one class loader, and libraries are never
unloaded. This means some care must be exercised to make sure that
a gcjlib URL is not accessed by more than one class loader at once.
In a future release this limitation will be lifted, and such
libraries will be mapped privately.
gcj will examine the
GCJ_PROPERTIES environment variable and change its behavior in
some ways. In particular GCJ_PROPERTIES holds a list of
assignments to global properties, such as would be set with the
-D option to java. For instance,
java.compiler=gcj is a valid (but currently meaningless)
setting.
The gcjh program is used to generate header files from class
files. It can generate both CNI and JNI header files, as well as stub
implementation files which can be used as a basis for implementing the
required native methods.
-stubs
gcjh to generate stub files instead of header files.
By default the stub file will be named after the class, with a suffix of
.cc. In JNI mode, the default output file will have the suffix
.c.
-jni
gcjh to generate a JNI header or stub. By default,
CNI headers are generated.
-add text-append text-friend textfriend declaration.
This is ignored in JNI mode.
-prepend text--classpath=path--CLASSPATH=path-Idirectory-d directory-o filegcj options.
-o file-td directory-M
-MM
-MD
-MMD
--help
gcjh and exit. No further processing is done.
--version
gcjh and exit. No further
processing is done.
-v, --verbose
All remaining options are considered to be names of classes.
The jv-scan program can be used to print information about a Java
source file (.java file).
--no-assert
assert keyword, for backwards compatibility
with older versions of the language specification.
--complexity
--encoding=namegcj option.
--print-main
main
method.
--list-class
--list-filename
--list-class is given, this option causes jv-scan to
also print the name of the file in which each class was found.
-o file--help
--version
This is a class file examiner, similar to javap. It will print
information about a number of classes, which are specified by class name
or file name.
-c
--javap
javap format. The implementation of this
feature is very incomplete.
--classpath=path--CLASSPATH=path-Idirectory-o filegcj options.
--help
--version
-v, --verbose
gij is a Java bytecode interpreter included with libgcj.
gij is not available on every platform; porting it requires a
small amount of assembly programming which has not been done for all the
targets supported by gcj.
The primary argument to gij is the name of a class or, with
-jar, a jar file. Options before this argument are interpreted
by gij; remaining options are passed to the interpreted program.
If a class name is specified and this class does not have a main
method with the appropriate signature (a static void method with
a String[] as its sole argument), then gij will print an
error and exit.
If a jar file is specified then gij will use information in it to
determine which class' main method will be invoked.
gij will invoke the main method with all the remaining
command-line options.
Note that gij is not limited to interpreting code. Because
libgcj includes a class loader which can dynamically load shared
objects, it is possible to give gij the name of a class which has
been compiled and put into a shared library on the class path.
-cp path-classpath pathCLASSPATH environment variable. Note that this option is
ignored if -jar is used.
-Dname[=value]
java.lang.System.getProperty
method.
-ms=number-mx=number-X
-Xargument-X by itself will cause gij to list all the
supported -X options. Currently there are none. Unrecognized
-X options are ignored, for compatibility with other runtimes.
-jar
gij should be interpreted
as the name of a jar file, not a class.
--help
-?
--showversion
--version
jv-convert [OPTION] ... [INPUTFILE [OUTPUTFILE]]
jv-convert is a utility included with libgcj which
converts a file from one encoding to another. It is similar to the Unix
iconv utility.
The encodings supported by jv-convert are platform-dependent.
Currently there is no way to get a list of all supported encodings.
--encoding name--from name--to nameJavaSrc encoding; this is ASCII with \u escapes for
non-ASCII characters.
-i file-o file--reverse
--help
--version
rmic [OPTION] ... class ...
rmic is a utility included with libgcj which generates
stubs for remote objects.
Note that this program isn't yet fully compatible with the JDK
rmic. Some options, such as -classpath, are
recognized but currently ignored. We have left these options
undocumented for now.
Long options can also be given with a GNU-style leading --. For
instance, --help is accepted.
-keep
-keepgenerated
rmic deletes intermediate files. Either of these
options causes it not to delete such files.
-v1.1
rmic to create stubs and skeletons for the 1.1
protocol version.
-vcompat
rmic to create stubs and skeletons compatible with both
the 1.1 and 1.2 protocol versions. This is the default.
-v1.2
rmic to create stubs and skeletons for the 1.2
protocol version.
-nocompile
-verbose
rmic is doing.
-d directory-help
-version
rmic [OPTION] ... [port]
rmiregistry starts a remote object registry on the current
host. If no port number is specified, then port 1099 is used.
--help
--version
This documents CNI, the Compiled Native Interface, which is is a convenient way to write Java native methods using C++. This is a more efficient, more convenient, but less portable alternative to the standard JNI (Java Native Interface).
In terms of languages features, Java is mostly a subset of C++. Java has a few important extensions, plus a powerful standard class library, but on the whole that does not change the basic similarity. Java is a hybrid object-oriented language, with a few native types, in addition to class types. It is class-based, where a class may have static as well as per-object fields, and static as well as instance methods. Non-static methods may be virtual, and may be overloaded. Overloading is resolved at compile time by matching the actual argument types against the parameter types. Virtual methods are implemented using indirect calls through a dispatch table (virtual function table). Objects are allocated on the heap, and initialized using a constructor method. Classes are organized in a package hierarchy.
All of the listed attributes are also true of C++, though C++ has
extra features (for example in C++ objects may be allocated not just
on the heap, but also statically or in a local stack frame). Because
gcj uses the same compiler technology as G++ (the GNU
C++ compiler), it is possible to make the intersection of the two
languages use the same ABI (object representation and calling
conventions). The key idea in CNI is that Java objects are C++
objects, and all Java classes are C++ classes (but not the other way
around). So the most important task in integrating Java and C++ is to
remove gratuitous incompatibilities.
You write CNI code as a regular C++ source file. (You do have to use a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
A CNI C++ source file must have:
#include <gcj/cni.h>
and then must include one header file for each Java class it uses, e.g.:
#include <java/lang/Character.h>
#include <java/util/Date.h>
#include <java/lang/IndexOutOfBoundsException.h>
These header files are automatically generated by gcjh.
CNI provides some functions and macros to make using Java objects and
primitive types from C++ easier. In general, these CNI functions and
macros start with the Jv prefix, for example the function
JvNewObjectArray. This convention is used to avoid conflicts
with other libraries. Internal functions in CNI start with the prefix
_Jv_. You should not call these; if you find a need to, let us
know and we will try to come up with an alternate solution. (This
manual lists _Jv_AllocBytes as an example; CNI should instead
provide a JvAllocBytes function.)
Whilst a Java class is just a C++ class that doesn't mean that you are freed from the shackles of Java, a CNI C++ class must adhere to the rules of the Java programming language.
For example: it is not possible to declare a method in a CNI class
that will take a C string (char*) as an argument, or to declare a
member variable of some non-Java datatype.
The only global names in Java are class names, and packages. A package can contain zero or more classes, and also zero or more sub-packages. Every class belongs to either an unnamed package or a package that has a hierarchical and globally unique name.
A Java package is mapped to a C++ namespace. The Java class
java.lang.String is in the package java.lang, which is a
sub-package of java. The C++ equivalent is the class
java::lang::String, which is in the namespace java::lang
which is in the namespace java.
Here is how you could express this:
(// Declare the class(es), possibly in a header file:
namespace java {
namespace lang {
class Object;
class String;
...
}
}
class java::lang::String : public java::lang::Object
{
...
};
The gcjh tool automatically generates the necessary namespace
declarations.
Always using the fully-qualified name of a java class can be
tiresomely verbose. Using the full qualified name also ties the code
to a single package making code changes necessary should the class
move from one package to another. The Java package declaration
specifies that the following class declarations are in the named
package, without having to explicitly name the full package
qualifiers. The package declaration can be
followed by zero or more import declarations, which
allows either a single class or all the classes in a package to be
named by a simple identifier. C++ provides something similar with the
using declaration and directive.
In Java:
import package-name.class-name;
allows the program text to refer to class-name as a shorthand for
the fully qualified name: package-name.class-name.
To achieve the same effect C++, you have to do this:
using package-name::class-name;
Java can also cause imports on demand, like this:
import package-name.*;
Doing this allows any class from the package package-name to be referred to only by its class-name within the program text.
The same effect can be achieved in C++ like this:
using namespace package-name;
Java provides 8 primitives types which represent integers, floats, characters and booleans (and also the void type). C++ has its own very similar concrete types. Such types in C++ however are not always implemented in the same way (an int might be 16, 32 or 64 bits for example) so CNI provides a special C++ type for each primitive Java type:
| Java type | C/C++ typename | Description
|
char | jchar | 16 bit Unicode character
|
boolean | jboolean | logical (true or false) values
|
byte | jbyte | 8-bit signed integer
|
short | jshort | 16 bit signed integer
|
int | jint | 32 bit signed integer
|
long | jlong | 64 bit signed integer
|
float | jfloat | 32 bit IEEE floating point number
|
double | jdouble | 64 bit IEEE floating point number
|
void | void | no value
|
When referring to a Java type You should always use these C++ typenames (e.g.: jint)
to avoid disappointment.
In Java each primitive type has an associated reference type,
e.g.: boolean has an associated java.lang.Boolean class.
In order to make working with such classes easier GCJ provides the macro
JvPrimClass:
| JvPrimClass type | macro |
Return a pointer to the Class object corresponding to the type supplied.
JvPrimClass(void) => java.lang.Void.TYPE
|
A Java class can implement zero or more interfaces, in addition to inheriting from a single base class.
CNI allows CNI code to implement methods of interfaces. You can also call methods through interface references, with some limitations.
CNI doesn't understand interface inheritance at all yet. So, you can only call an interface method when the declared type of the field being called matches the interface which declares that method. The workaround is to cast the interface reference to the right superinterface.
For example if you have:
interface A
{
void a();
}
interface B extends A
{
void b();
}
and declare a variable of type B in C++, you can't call
a() unless you cast it to an A first.
All Java classes are derived from java.lang.Object. C++ does
not have a unique root class, but we use the C++ class
java::lang::Object as the C++ version of the
java.lang.Object Java class. All other Java classes are mapped
into corresponding C++ classes derived from java::lang::Object.
Interface inheritance (the implements keyword) is currently not
reflected in the C++ mapping.
Each object contains an object header, followed by the instance fields of the class, in order. The object header consists of a single pointer to a dispatch or virtual function table. (There may be extra fields in front of the object, for example for memory management, but this is invisible to the application, and the reference to the object points to the dispatch table pointer.)
The fields are laid out in the same order, alignment, and size as in
C++. Specifically, 8-bite and 16-bit native types (byte,
short, char, and boolean) are not widened
to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
to 32 bits when on the VM stack or temporary registers.
If you include the gcjh-generated header for a
class, you can access fields of Java classes in the natural
way. For example, given the following Java class:
public class Int
{
public int i;
public Integer (int i) { this.i = i; }
public static zero = new Integer(0);
}
you can write:
#include <gcj/cni.h>;
#include <Int>;
Int*
mult (Int *p, jint k)
{
if (k == 0)
return Int::zero; // Static member access.
return new Int(p->i * k);
}
CNI does not strictly enforce the Java access specifiers, because Java permissions cannot be directly mapped into C++ permission. Private Java fields and methods are mapped to private C++ fields and methods, but other fields and methods are mapped to public fields and methods.
Java requires that each class be automatically initialized at the time
of the first active use. Initializing a class involves
initializing the static fields, running code in class initializer
methods, and initializing base classes. There may also be
some implementation specific actions, such as allocating
String objects corresponding to string literals in
the code.
The GCJ compiler inserts calls to JvInitClass at appropriate
places to ensure that a class is initialized when required. The C++
compiler does not insert these calls automatically--it is the
programmer's responsibility to make sure classes are initialized.
However, this is fairly painless because of the conventions assumed by
the Java system.
First, libgcj will make sure a class is initialized before an
instance of that object is created. This is one of the
responsibilities of the new operation. This is taken care of
both in Java code, and in C++ code. When G++ sees a new of a
Java class, it will call a routine in libgcj to allocate the
object, and that routine will take care of initializing the class.
Note however that this does not happen for Java arrays; you must
allocate those using the appropriate CNI function. It follows that
you can access an instance field, or call an instance (non-static)
method and be safe in the knowledge that the class and all of its base
classes have been initialized.
Invoking a static method is also safe. This is because the
Java compiler adds code to the start of a static method to make sure
the class is initialized. However, the C++ compiler does not
add this extra code. Hence, if you write a native static method
using CNI, you are responsible for calling JvInitClass
before doing anything else in the method (unless you are sure
it is safe to leave it out).
Accessing a static field also requires the class of the
field to be initialized. The Java compiler will generate code
to call Jv_InitClass before getting or setting the field.
However, the C++ compiler will not generate this extra code,
so it is your responsibility to make sure the class is
initialized before you access a static field from C++.
New Java objects are allocated using a class instance creation expression, e.g.:
new Type ( ... )
The same syntax is used in C++. The main difference is that C++ objects have to be explicitly deleted; in Java they are automatically deleted by the garbage collector. Using CNI, you can allocate a new Java object using standard C++ syntax and the C++ compiler will allocate memory from the garbage collector. If you have overloaded constructors, the compiler will choose the correct one using standard C++ overload resolution rules.
For example:
java::util::Hashtable *ht = new java::util::Hashtable(120);
| void* _Jv_AllocBytes (jsize size) | Function |
| Allocates size bytes from the heap. The memory is not scanned by the garbage collector but it freed if no references to it are discovered. |
While in many ways Java is similar to C and C++, it is quite different
in its treatment of arrays. C arrays are based on the idea of pointer
arithmetic, which would be incompatible with Java's security
requirements. Java arrays are true objects (array types inherit from
java.lang.Object). An array-valued variable is one that
contains a reference (pointer) to an array object.
Referencing a Java array in C++ code is done using the
JArray template, which as defined as follows:
class __JArray : public java::lang::Object
{
public:
int length;
};
template<class T>
class JArray : public __JArray
{
T data[0];
public:
T& operator[](jint i) { return data[i]; }
};
There are a number of typedefs which correspond to typedefs
from the JNI. Each is the type of an array holding objects
of the relevant type:
typedef __JArray *jarray;
typedef JArray<jobject> *jobjectArray;
typedef JArray<jboolean> *jbooleanArray;
typedef JArray<jbyte> *jbyteArray;
typedef JArray<jchar> *jcharArray;
typedef JArray<jshort> *jshortArray;
typedef JArray<jint> *jintArray;
typedef JArray<jlong> *jlongArray;
typedef JArray<jfloat> *jfloatArray;
typedef JArray<jdouble> *jdoubleArray;
| T* elements (JArray |
Method on template<class T> |
This template function can be used to get a pointer to the elements of
the array. For instance, you can fetch a pointer to the
integers that make up an int[] like so:
extern jintArray foo;
jint *intp = elements (foo);
The name of this function may change in the future. |
| jobjectArray JvNewObjectArray (jsize length, jclass klass, jobject init) | Function |
Here klass is the type of elements of the array and
init is the initial value put into every slot in the array.
|
For each primitive type there is a function which can be used to create a new array of that type. The name of the function is of the form:
JvNewTypeArray
For example:
JvNewBooleanArray
can be used to create an array of Java primitive boolean types.
The following function definition is the template for all such functions:
| jbooleanArray JvNewBooleanArray (jint length) | Function |
| Create's an array length indices long. |
| jsize JvGetArrayLength (jarray array) | Function |
| Returns the length of the array. |
Java methods are mapped directly into C++ methods.
The header files generated by gcjh
include the appropriate method definitions.
Basically, the generated methods have the same names and
corresponding types as the Java methods,
and are called in the natural manner.
Both Java and C++ provide method overloading, where multiple
methods in a class have the same name, and the correct one is chosen
(at compile time) depending on the argument types.
The rules for choosing the correct method are (as expected) more complicated
in C++ than in Java, but given a set of overloaded methods
generated by gcjh the C++ compiler will choose
the expected one.
Common assemblers and linkers are not aware of C++ overloading, so the standard implementation strategy is to encode the parameter types of a method into its assembly-level name. This encoding is called mangling, and the encoded name is the mangled name. The same mechanism is used to implement Java overloading. For C++/Java interoperability, it is important that both the Java and C++ compilers use the same encoding scheme.
Static Java methods are invoked in CNI using the standard
C++ syntax, using the :: operator rather
than the . operator.
For example:
jint i = java::lang::Math::round((jfloat) 2.3);
C++ method definition syntax is used to define a static native method. For example:
#include <java/lang/Integer>
java::lang::Integer*
java::lang::Integer::getInteger(jstring str)
{
...
}
Constructors are called implicitly as part of object allocation
using the new operator.
For example:
java::lang::Integer *x = new java::lang::Integer(234);
Java does not allow a constructor to be a native method. This limitation can be coded round however because a constructor can call a native method.
Calling a Java instance method from a C++ CNI method is done using the standard C++ syntax, e.g.:
// First create the Java object.
java::lang::Integer *x = new java::lang::Integer(234);
// Now call a method.
jint prim_value = x->intValue();
if (x->longValue == 0)
...
Defining a Java native instance method is also done the natural way:
#include <java/lang/Integer.h>
jdouble
java::lang:Integer::doubleValue()
{
return (jdouble) value;
}
In Java you can call a method using an interface reference. This is supported, but not completely. See Interfaces.
CNI provides a number of utility functions for
working with Java Java String objects.
The names and interfaces are analogous to those of JNI.
| jstring JvNewString (const char* chars, jsize len) | Function |
Returns a Java String object with characters from the C string
chars up to the index len in that array.
|
| jstring JvNewStringLatin1 (const char* bytes, jsize len) | Function |
Returns a Java String made up of len bytes from bytes.
|
| jstring JvNewStringLatin1 (const char* bytes) | Function |
As above but the length of the String is strlen(bytes).
|
| jstring JvNewStringUTF (const char* bytes) | Function |
Returns a String which is made up of the UTF encoded characters
present in the C string bytes.
|
| jchar* JvGetStringChars (jstring str) | Function |
Returns a pointer to an array of characters making up the String str.
|
| int JvGetStringUTFLength (jstring str) | Function |
Returns the number of bytes required to encode the contents of the
String str in UTF-8.
|
| jsize JvGetStringUTFRegion (jstring str, jsize start, jsize len, char* buf) | Function |
Puts the UTF-8 encoding of a region of the String str into
the buffer buf. The region to fetch is marked by start and len.
Note that buf is a buffer, not a C string. It is not null terminated. |
Because CNI is designed to represent Java classes and methods it cannot be mixed readily with C/C++ types.
One important restriction is that Java classes cannot have non-Java type instance or static variables and cannot have methods which take non-Java types as arguments or return non-Java types.
None of the following is possible with CNI:
class ::MyClass : public java::lang::Object
{
char* variable; // char* is not a valid Java type.
}
uint
::SomeClass::someMethod (char *arg)
{
.
.
.
} // uint is not a valid Java type, neither is char*
Of course, it is ok to use C/C++ types within the scope of a method:
jint
::SomeClass::otherMethod (jstring str)
{
char *arg = ...
.
.
.
}
But this restriction can cause a problem so CNI includes the
gnu.gcj.RawData class. The RawData class is a
non-scanned reference type. In other words variables declared
of type RawData can contain any data and are not checked by the
compiler in any way.
This means that you can put C/C++ data structures (including classes) in your CNI classes, as long as you use the appropriate cast.
Here are some examples:
class ::MyClass : public java::lang::Object
{
gnu.gcj.RawData string;
MyClass ();
gnu.gcj.RawData getText ();
void printText ();
}
::MyClass::MyClass ()
{
char* text = ...
string = text;
}
gnu.gcj.RawData
::MyClass::getText ()
{
return string;
}
void
::MyClass::printText ()
{
printf("%s\n", (char*) string);
}
While C++ and Java share a common exception handling framework, things are not yet perfectly integrated. The main issue is that the run-time type information facilities of the two languages are not integrated.
Still, things work fairly well. You can throw a Java exception from
C++ using the ordinary throw construct, and this
exception can be caught by Java code. Similarly, you can catch an
exception thrown from Java using the C++ catch
construct.
Here is an example:
if (i >= count)
throw new java::lang::IndexOutOfBoundsException();
Normally, G++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code:
struct S { ~S(); };
extern void bar(); // Is implemented in Java and may throw exceptions.
void foo()
{
S s;
bar();
}
The usual effect of an incorrect guess is a link failure, complaining of
a missing routine called __gxx_personality_v0.
You can inform the compiler that Java exceptions are to be used in a
translation unit, irrespective of what it might think, by writing
#pragma GCC java_exceptions at the head of the
file. This #pragma must appear before any
functions that throw or catch exceptions, or run destructors when
exceptions are thrown through them.
Each Java object has an implicit monitor.
The Java VM uses the instruction monitorenter to acquire
and lock a monitor, and monitorexit to release it.
The corresponding CNI macros are JvMonitorEnter and
JvMonitorExit (JNI has similar methods MonitorEnter
and MonitorExit).
The Java source language does not provide direct access to these primitives.
Instead, there is a synchronized statement that does an
implicit monitorenter before entry to the block,
and does a monitorexit on exit from the block.
Note that the lock has to be released even when the block is abnormally
terminated by an exception, which means there is an implicit
try finally surrounding synchronization locks.
From C++, it makes sense to use a destructor to release a lock. CNI defines the following utility class:
class JvSynchronize() {
jobject obj;
JvSynchronize(jobject o) { obj = o; JvMonitorEnter(o); }
~JvSynchronize() { JvMonitorExit(obj); }
};
So this Java code:
synchronized (OBJ)
{
CODE
}
might become this C++ code:
{
JvSynchronize dummy (OBJ);
CODE;
}
Java also has methods with the synchronized attribute.
This is equivalent to wrapping the entire method body in a
synchronized statement.
(Alternatively, an implementation could require the caller to do
the synchronization. This is not practical for a compiler, because
each virtual method call would have to test at run-time if
synchronization is needed.) Since in gcj
the synchronized attribute is handled by the
method implementation, it is up to the programmer
of a synchronized native method to handle the synchronization
(in the C++ implementation of the method).
In other words, you need to manually add JvSynchronize
in a native synchronized method.
CNI permits C++ applications to make calls into Java classes, in addition to allowing Java code to call into C++. Several functions, known as the invocation API, are provided to support this.
| jint JvCreateJavaVM (void* vm_args) | Function |
Initializes the Java runtime. This function performs essential initialization
of the threads interface, garbage collector, exception handling and other key
aspects of the runtime. It must be called once by an application with
a non-Java main() function, before any other Java or CNI calls are made.
It is safe, but not recommended, to call JvCreateJavaVM() more than
once provided it is only called from a single thread.
The vmargs parameter can be used to specify initialization parameters
for the Java runtime. It may be NULL.
This function returns 0 upon success, or -1 if the runtime is
already initialized.
Note: In GCJ 3.1, the |
| java::lang::Thread* JvAttachCurrentThread (jstring name, java::lang::ThreadGroup* group) | Function |
Registers an existing thread with the Java runtime. This must be called once
from each thread, before that thread makes any other Java or CNI calls. It
must be called after JvCreateJavaVM.
name specifies a name for the thread. It may be NULL, in which
case a name will be generated.
group is the ThreadGroup in which this thread will be a member. If it
is NULL, the thread will be a member of the main thread group.
The return value is the Java Thread object that represents the thread.
It is safe to call JvAttachCurrentThread() more than once from the same
thread. If the thread is already attached, the call is ignored and the current
thread object is returned.
|
| jint JvDetachCurrentThread () | Function |
Unregisters a thread from the Java runtime. This should be called by threads
that were attached using JvAttachCurrentThread(), after they have
finished making calls to Java code. This ensures that any resources associated
with the thread become eligible for garbage collection.
This function returns 0 upon success, or -1 if the current thread
is not attached.
|
If an exception is thrown from Java code called using the invocation API, and no handler for the exception can be found, the runtime will abort the application. In order to make the application more robust, it is recommended that code which uses the invocation API be wrapped by a top-level try/catch block that catches all Java exceptions.
The following code demonstrates the use of the invocation API. In this
example, the C++ application initializes the Java runtime and attaches
itself. The java.lang.System class is initialized in order to
access its out field, and a Java string is printed. Finally, the thread
is detached from the runtime once it has finished making Java calls. Everything
is wrapped with a try/catch block to provide a default handler for any uncaught
exceptions.
The example can be compiled with c++ test.cc -lgcj.
// test.cc
#include <gcj/cni.h>
#include <java/lang/System.h>
#include <java/io/PrintStream.h>
#include <java/lang/Throwable.h>
int main(int argc, char *argv)
{
using namespace java::lang;
try
{
JvCreateJavaVM(NULL);
JvAttachCurrentThread(NULL, NULL);
String *message = JvNewStringLatin1("Hello from C++");
JvInitClass(&System::class$);
System::out->println(message);
JvDetachCurrentThread();
}
catch (Throwable *t)
{
System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
t->printStackTrace();
}
}
Reflection is possible with CNI code, it functions similarly to how it functions with JNI.
The types jfieldID and jmethodID
are as in JNI.
The functions:
JvFromReflectedField,
JvFromReflectedMethod,
JvToReflectedField
JvToFromReflectedMethod
will be added shortly, as will other functions corresponding to JNI.
The runtime behavior of the libgcj library can be modified by setting
certain system properties. These properties can be compiled into the program
using the -Dname[=value] option to gcj or by
setting them explicitly in the program by calling the
java.lang.System.setProperty() method. Some system properties are only
used for informational purposes (like giving a version number or a user name).
A program can inspect the current value of a property by calling the
java.lang.System.getProperty() method.
The following properties are normally found in all implementations of the core libraries for the Java language.
java.version
libgcj version number.
java.vendor
The Free Software Foundation, Inc.
java.vendor.url
java.home
gcj was installed. Taken from the --prefix
option given to configure.
java.class.version
46.0)
java.vm.specification.version
libgcj.
(Currently 1.0)
java.vm.specification.vendor
java.vm.specification.name
Java Virtual Machine Specification).
java.vm.version
gcj version number.
java.vm.vendor
The Free Software Foundation, Inc.
java.vm.name
GNU libgcj.
java.specification.version
libgcj.
(Currently set to 1.3)
java.specification.vendor
java.specification.name
Java Platform API Specification).
java.class.path
java.library.path
java.io.tmpdir
java.compiler
libgcj.
java.ext.dirs
libgcj.
java.protocol.handler.pkgs
| separated list of package names that is used to find classes that
implement handlers for java.net.URL.
java.rmi.server.codebase
java.rmi.server.RMIClassLoader
to load classes from.
jdbc.drivers
java.sql.DriverManager
when it starts up.
file.separator
/ or \ ).
file.encoding
8859_1).
path.separator
: or ;), the string is usually not a valid character
to use in normal directory names.)
line.separator
\n, \r
or a combination of those two characters).
policy.provider
java.security.Policy.getPolicy.
user.name
user.home
user.dir
user.language
java.util.Locale class.
user.region
java.util.Local class.
user.variant
user.timezone
java.util.TimeZone class.
os.name
os.arch
os.version
awt.appletWarning
java.awt.Window.getWarningString() when the window is
"insecure".
awt.toolkit
java.awt.Toolkit.
Defaults to gnu.awt.gtk.GtkToolkit.
http.proxyHost
http.proxyPort
libgcj is based on the GNU Classpath (Essential Libraries for Java) a
GNU project to create free core class libraries for use with virtual machines
and compilers for the Java language. The following properties are common to
libraries based on GNU Classpath.
gcj.dumpobject
java.io.ObjectInput and
java.io.ObjectOutput classes when set to something else then the empty
string. Only used when running a debug build of the library.
gnu.classpath.vm.shortname
libgcj,
this will always be libgcj.
gnu.classpath.home.url
classpath.security). By default this is a file: URL
pointing to the lib directory under java.home.
The following properties are specific to the libgcj runtime and will
normally not be found in other core libraries for the java language.
java.fullversion
java.vm.name and java.vm.version.
java.vm.info
java.fullversion.
impl.prefix
java.net.DatagramSocket class when set to something else
then the empty string. When set all newly created DatagramSockets will
try to load a class java.net.[impl.prefix]DatagramSocketImpl instead of
the normal java.net.PlainDatagramSocketImpl.
gnu.gcj.progname
gnu.gcj.runtime.NameFinder.demangle
true.
gnu.gcj.runtime.NameFinder.sanitize
true.
gnu.gcj.runtime.NameFinder.remove_unknown
true.
gnu.gcj.runtime.NameFinder.remove_interpreter
_Jv_InterpMethod class
and functions starting with ffi_) should be removed from the stack
trace. Only done when the stack is sanitized. Defaults to true.
gnu.gcj.runtime.NameFinder.use_addr2line
addr2line or addr2name.awk)
should be used as fallback to convert the addresses to function names when
the runtime is unable to do it through dladdr.
gnu.gcj.runtime.VMClassLoader.library_control
full, a full search is done for each requested class. If this
property is set to cache, then any failed lookups are cached
and not tried again. If this property is set to never, then
lookups are never done. For more information, See Extensions.
While writing gcj and libgcj we have, of course, relied
heavily on documentation from Sun Microsystems. In particular we have
used The Java Language Specification (both first and second editions),
the Java Class Libraries (volumes one and two), and the Java Virtual
Machine Specification. In addition we've used the online documentation
at http://java.sun.com/.
The current gcj home page is
http://gcc.gnu.org/java/.
For more information on gcc, see http://gcc.gnu.org/.
Some libgcj testing is done using the Mauve test suite. This is
a free software Java class library test suite which is being written
because the JCK is not free. See
http://sources.redhat.com/mauve/ for more information.
The linker by default looks for a global function named
main. Since Java does not have global functions, and a
collection of Java classes may have more than one class with a
main method, you need to let the linker know which of those
main methods it should invoke when starting the application.