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); }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; }文章来源: Java下FileChannel的实现剖析