Java下FileChannel的实现剖析

源码剖析

FileChannel实现

openjdk\jdk\src\share\classes\sun\nio\ch\FileChannelImpl.java
先尝试sendfile，如操作系统不支持，对于信任的channel类型，调用mmap，否则走标准的read/write系统调用。windows系统是不支持sendfile的，所以windows下实际走的是mmap，性能较之read/write方式也要好很多。
从源码来看，只有源为FileChannel才支持transfer这种高效的复制方式，其他如SocketChannel都不支持transfer模式。当然，目的Channel没有这种限制。所以一般可以做FileChannel->FileChannel和FileChannel->SocketChannel的transfer。

public long transferTo(long position, long count,
                           WritableByteChannel target)
        throws IOException
    {
        ensureOpen();
        if (!target.isOpen())
            throw new ClosedChannelException();
        if (!readable)
            throw new NonReadableChannelException();
        if (target instanceof FileChannelImpl &&
            !((FileChannelImpl)target).writable)
            throw new NonWritableChannelException();
        if ((position < 0) || (count < 0))
            throw new IllegalArgumentException();
        long sz = size();
        if (position > sz)
            return 0;
        int icount = (int)Math.min(count, Integer.MAX_VALUE);
        if ((sz - position) < icount)
            icount = (int)(sz - position);

        long n;

        // Attempt a direct transfer, if the kernel supports it
        if ((n = transferToDirectly(position, icount, target)) >= 0)
            return n;

        // Attempt a mapped transfer, but only to trusted channel types
        if ((n = transferToTrustedChannel(position, icount, target)) >= 0)
            return n;

        // Slow path for untrusted targets
        return transferToArbitraryChannel(position, icount, target);
    }

对sendfile的支持

openjdk\jdk\src\solaris\native\sun\nio\ch\FileChannelImpl.c
linux是sendfile64（我的是64位的java版本）：

JNIEXPORT jlong JNICALL
Java_sun_nio_ch_FileChannelImpl_transferTo0(JNIEnv *env, jobject this,
                                            jint srcFD,
                                            jlong position, jlong count,
                                            jint dstFD)
{
#if defined(__linux__)
    off64_t offset = (off64_t)position;
    jlong n = sendfile64(dstFD, srcFD, &offset, (size_t)count);
    if (n < 0) {
        if (errno == EAGAIN)
            return IOS_UNAVAILABLE;
        if ((errno == EINVAL) && ((ssize_t)count >= 0))
            return IOS_UNSUPPORTED_CASE;
        if (errno == EINTR) {
            return IOS_INTERRUPTED;
        }
        JNU_ThrowIOExceptionWithLastError(env, "Transfer failed");
        return IOS_THROWN;
    }
    return n;
    ......

openjdk\jdk\src\windows\native\sun\nio\ch\FileChannelImpl.c
windows是不支持的：

JNIEXPORT jlong JNICALL
Java_sun_nio_ch_FileChannelImpl_transferTo0(JNIEnv *env, jobject this,
                                            jint srcFD,
                                            jlong position, jlong count,
                                            jint dstFD)
{
    return IOS_UNSUPPORTED;
}