How to change Java native code (C implementation) from the JDK (SocketChannelImpl and SocketDispatcher) and recompile to use with the JDK?

Question

I need to locate, recompile and deploy on my JDK running on Linux the following native methods below, from SocketDispatcher.java: (Native methods are on the bottom of the code below, so scroll it down please)

package sun.nio.ch;

import java.io.*;

/**
 * Allows different platforms to call different native methods
 * for read and write operations.
 */

class SocketDispatcher extends NativeDispatcher
{

    static {
        IOUtil.load();
    }

    int read(FileDescriptor fd, long address, int len) throws IOException {
        return read0(fd, address, len);
    }

    long readv(FileDescriptor fd, long address, int len) throws IOException {
        return readv0(fd, address, len);
    }

    int write(FileDescriptor fd, long address, int len) throws IOException {
        return write0(fd, address, len);
    }

    long writev(FileDescriptor fd, long address, int len) throws IOException {
        return writev0(fd, address, len);
    }

    void preClose(FileDescriptor fd) throws IOException {
        preClose0(fd);
    }

    void close(FileDescriptor fd) throws IOException {
        close0(fd);
    }

    //-- Native methods
    static native int read0(FileDescriptor fd, long address, int len)
        throws IOException;

    static native long readv0(FileDescriptor fd, long address, int len)
        throws IOException;

    static native int write0(FileDescriptor fd, long address, int len)
        throws IOException;

    static native long writev0(FileDescriptor fd, long address, int len)
        throws IOException;

    static native void preClose0(FileDescriptor fd) throws IOException;

    static native void close0(FileDescriptor fd) throws IOException;
}

Source: http://grepcode.com/file_/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/sun/nio/ch/SocketDispatcher.java/?v=source

So basically I want to make the native method write0 print "Hello from native write0!".

Questions:

Where is the C/C++ code for write0 for Linux?
How do I recompile (gcc command-line?) the C/C++ code for write0 on Linux?
How do I start my JVM and make it use my new compiled native code for write0 on Linux?

score 6 · Answer 1 · edited May 23 '17 at 12:32

6

There is no native SocketDispatcher implementation on Linux. It is Windows-specific class; on Linux it just shares the implementation with FileDispatcherImpl.

The native code for FileDispatcherImpl is in src/solaris/native/sun/nio/ch/FileDispatcherImpl.c (in OpenJDK solaris directory stands for POSIX code, so Linux implementation is also there).

If you want to override native methods, you have to create a shared library with your own implementation. Native method names follow JNI conventions, e.g. the native function for FileDispatherImpl.write0 should have the following signature:

JNIEXPORT jint JNICALL
Java_sun_nio_ch_FileDispatcherImpl_write0(JNIEnv *env, jclass clazz,
                              jobject fdo, jlong address, jint len)

Once you've built your own shared library with one or more functions overriden, preload it with LD_PRELOAD, and JVM will link your functions before any native code from JDK. By the way, JDK natives for FileDispatcherImpl and other java.nio stuff comes in libnio.so.

edited May 23 '17 at 12:32

Community

1
1

answered Apr 12 '17 at 08:07

apangin

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Thanks, but I would like to re-compile the file `src/solaris/native/sun/nio/ch/FileDispatcherImpl.c` instead of writing my own implementation from scratch. So I want to copy everything from FileDispatcher.c change some small things, recompile and override. – LatencyFighter Apr 12 '17 at 13:37
@LatencyFighter Then you'll have to build the whole OpenJDK. – apangin Apr 12 '17 at 14:02
I just want to recompile the code for that particular method to print to the console a "Hello from write!" message before the method does what it already does. I can use the `LD_PRELOAD`. I guess the challenge there would be **How to compile?**, what command line to use, what dependencies it needs, etc. – LatencyFighter Apr 12 '17 at 14:36
@LatencyFighter You don't need to copy the original source file if you just want to *prepend* some action. It is possible to chain the original library from your LD_PRELOAD'ed library by calling `dlsym(RTLD_NEXT, ...)` – apangin Apr 12 '17 at 15:06
1

`gcc -shared -fPIC -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -olibmynio.so mynio.c -ldl` – apangin Apr 12 '17 at 15:06

How to change Java native code (C implementation) from the JDK (SocketChannelImpl and SocketDispatcher) and recompile to use with the JDK?

1 Answers1