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;
}