如何从JDK（SocketChannelImpl和SocketDispatcher）更改Java本机代码（C实现）并重新编译以与JDK一起使用？

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?

Answer 1

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 , eg 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 .