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* doc/cni.sgml: Updated from master copy.

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Tom Tromey 2000-09-05 17:48:57 +00:00 committed by Tom Tromey
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libjava/ChangeLog
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@ -1,3 +1,7 @@
2000-09-05 Tom Tromey <tromey@cygnus.com>
* doc/cni.sgml: Updated from master copy.
2000-09-05 Bryce McKinlay <bryce@albatross.co.nz>
* gnu/gcj/convert/natIconv.cc (read): Remove unused local.

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libjava/doc/cni.sgml
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@ -6,7 +6,7 @@
<authorgroup>
<corpauthor>Cygnus Solutions</corpauthor>
</authorgroup>
<date>February, 1999</date>
<date>March, 2000</date>
</artheader>
<abstract><para>
@ -369,98 +369,6 @@ to private C++ fields and methods, but other fields and methods
are mapped to public fields and methods.
</para>
</sect2>
<sect2><title>Non-Java fields</title>
<para>
When you write a Java wrapper around an existing library, that library
will often allocate and manage its own data structures. These are
<quote>objects</quote> that are not Java <literal>Object</literal>s;
instead they are usually C <literal>struct</literal> instances.
Typically, you will write a Java class, and use native CNI methods
which call functions in the C library. The problem is how to get
from the Java wrapper object to the C <literal>struct</literal> instances.
The obvious solution is to add a field to the Java object that
points to the C structure. The problem is that there is no Java
type that we can give to this field.</para>
<para>The GCJ solution is to define a special dummy class
<literal>gnu.gcj.RawData</literal>. This can be used as the type for fields,
parameters, array elements, or local variables in Java code.
It means that the field or variable is a pointer to a non-Java object.
Nothing else is known about it, so it corresponds to a
<literal>(void*)</literal> declaration is C or C++ code.</para>
<para>
The garbage collector will ignore a field that has type
<literal>gnu.gcj.RawData</literal>. You are responsible for
freeing the C data structure when you are done with it, and
performing any necessary cleanups. In most cases, you should
use a <literal>finalize</literal> method, and have it call
the library's cleanup routine. Also, the C data structure
should not contain a pointer back to the Java object, since
the garbage collector will not know about the pointer.
If you need to save a pointer to a Java object inside some
non-Java data structure, you first need to <quote>pin</quote>
or <quote>globalize</quote> the pointer; there is no CNI function
to do this yet.
(From the point of view of the
implementation, a <literal>gnu.gcj.RawData</literal> value is
the same as an integer that has the same size as a pointer.)</para>
<para>
Here is an example where we create a Java wrapper around C stdio:
<programlisting>
import gnu.gcj.RawData;
public class StdioFile
{
private RawData file;
public StdioFile (RawData file) { this.file = file; }
public StdioFile (String name, String mode)
throws FileNotFoundException
{ init(name, mode); }
private native void init (String name, String mode)
throws FileNotFoundException;
public native int getc();
public native int close();
protected native void finalize();
}
</programlisting>
This is the CNI implementation:
<programlisting>
jint
StdioFile::getc()
{
return getc((FILE*) file);
}
jint
StdioFile::close()
{
return fclose((FILE*) file);
}
void
StdioFile::init(jstring name, jstring mode)
{
int cname_len = JvGetStringUTFLength (name);
int cmode_len = JvGetStringUTFLength (mode);
char cname[cname_len + 1];
char cmode[cmode_len + 1];
JvGetStringUTFRegion (name, 0, name->length(), cname);
JvGetStringUTFRegion (mode, 0, mode->length(), cmode);
cname[cname_len] = '\0';
cmode[cmode_len] = '\0';
file = (gnu::gcj::RawData*) fopen(cname, cmode);
if (file == NULL)
JvThrow(new java::lang::FileNotFoundException(name));
}
void
StdioFile::finalize()
{
fclose((FILE*) file);
}
</programlisting>
</sect2>
</sect1>
<sect1><title>Arrays</title>
@ -640,7 +548,8 @@ During 1999, G++ will switch to a new ABI that is compatible with
<acronym>gcj</acronym>. Some platforms (including Linux) have already
changed. On other platforms, you will have to pass
the <literal>-fvtable-thunks</literal> flag to g++ when
compiling <acronym>CNI</acronym> code.
compiling <acronym>CNI</acronym> code. Note that you must also compile
your C++ source code with <literal>-fno-rtti</literal>.
</para>
<para>
Calling a Java instance method in <acronym>CNI</acronym> is done
@ -850,33 +759,24 @@ initialized before you access a static field.</para>
<sect1><title>Exception Handling</title>
<para>
While C++ and Java share a common exception handling framework,
things are not quite as integrated as we would like, yet.
The main issue is the incompatible exception <emphasis>values</emphasis>,
and that the <quote>run-time type information</quote> facilities of the
two languages are not integrated.</para>
things are not yet perfectly integrated. The main issue is that the
<quote>run-time type information</quote> facilities of the two
languages are not integrated.</para>
<para>
Basically, this means that you cannot in C++ catch an exception
value (<classname>Throwable</classname>) thrown from Java code, nor
can you use <literal>throw</literal> on a Java exception value from C++ code,
and expect to be able to catch it in Java code.
We do intend to change this.</para>
Still, things work fairly well. You can throw a Java exception from
C++ using the ordinary <literal>throw</literal> construct, and this
exception can be caught by Java code. Similarly, you can catch an
exception thrown from Java using the C++ <literal>catch</literal>
construct.
<para>
You can throw a Java exception from C++ code by using
the <literal>JvThrow</literal> <acronym>CNI</acronym> function.
<funcsynopsis>
<funcdef>void <function>JvThrow</function></funcdef>
<paramdef>jobject <parameter>obj</parameter></paramdef>
</funcsynopsis>
Throws an exception <parameter>obj</parameter>, in a way compatible
with the Java exception-handling functions.
The class of <parameter>obj</parameter> must be a subclass of
<literal>Throwable</literal>.
Note that currently you cannot mix C++ catches and Java catches in
a single C++ translation unit. We do intend to fix this eventually.
</para>
<para>
Here is an example:
<programlisting>
if (i >= count)
JvThrow (new java::lang::IndexOutOfBoundsException());
throw new java::lang::IndexOutOfBoundsException();
</programlisting>
</para>
</sect1>
@ -1044,7 +944,14 @@ as the names of classes for which headers should be generated.</para>
<para>
gcjh will generate all the required namespace declarations and
<literal>#include</literal>'s for the header file.
In some situations, gcjh will generate simple inline member functions.</para>
In some situations, gcjh will generate simple inline member
functions. Note that, while gcjh puts <literal>#pragma
interface</literal> in the generated header file, you should
<emphasis>not</emphasis> put <literal>#pragma implementation</literal>
into your C++ source file. If you do, duplicate definitions of
inline functions will sometimes be created, leading to link-time
errors.
</para>
<para>
There are a few cases where gcjh will fail to work properly:</para>
<para>