直接内存
然后咱们先写个代码看看直接内存分配在哪个区域
import java.nio.ByteBuffer;import java.util.ArrayList;import java.util.List;/** * @author congzhou * @description: * @date: Created in 2019/2/19 21:57 */public class NativeHeapTest { public static void main(String[] args) throws InterruptedException { List byteBufferList = new ArrayList<>(); while (true) { ByteBuffer byteBuffer = ByteBuffer.allocateDirect(10240); byteBufferList.add(byteBuffer); Thread.sleep(100); } }}复制代码 watch命令观察变化内存
再来看一下ByteBuffer.allocateDirect(int capacity)
ByteBuffer.allocateDirect调用了Unsafe.allocateMemory(long var1)，该方法是个native方法
UNSAFE_ENTRY(jlong, Unsafe_AllocateMemory0(JNIEnv *env, jobject unsafe, jlong size)) { size_t sz = (size_t)size; sz = align_up(sz, HeapWordSize); void* x = os::malloc(sz, mtOther); return addr_to_java(x);} UNSAFE_END复制代码 主要就是os::malloc(sz, mtInternal)方法，该方法封装了C++的标准库std::malloc()
void* os::malloc(size_t size, MEMFLAGS flags) { return os::malloc(size, flags, CALLER_PC);}void* os::malloc(size_t size, MEMFLAGS memflags, const NativeCallStack& stack) {/*省略代码*/ u_char* ptr; ptr = (u_char*)::malloc(alloc_size);/*省略代码*/ // we do not track guard memory return MemTracker::record_malloc((address)ptr, size, memflags, stack, level);}复制代码 (u_char*)::malloc(alloc_size)就是C++标准库函数，传入空间大小返回内存地址。
那么堆内存怎么往外写
openJDK sun.nio.ch.IOUtil.write()，java.nio.channels.SocketChannel#write(java.nio.ByteBuffer)等方法底层实现
static int write(FileDescriptor fd, ByteBuffer src, long position, NativeDispatcher nd) throws IOException { if (src instanceof DirectBuffer) return writeFromNativeBuffer(fd, src, position, nd); // Substitute a native buffer int pos = src.position(); int lim = src.limit(); assert (pos <= lim); int rem = (pos <= lim ? lim - pos : 0); ByteBuffer bb = Util.getTemporaryDirectBuffer(rem); try { bb.put(src); bb.flip(); // Do not update src until we see how many bytes were written src.position(pos); int n = writeFromNativeBuffer(fd, bb, position, nd); if (n > 0) { // now update src src.position(pos + n); } return n; } finally { Util.offerFirstTemporaryDirectBuffer(bb); } }复制代码 上述代码直观的展示了java堆内存首先拷贝到了直接内存，然后再把地址传给I/O函数。
java GC三大类算法，除了标记清除，标记整理和复制算法都会移动对象，并且如果直接把java堆地址传给I/O函数则需要保证I/O操作过程中该块内存不变化，则需要暂停GC，所以JDK实现使用拷贝的方式。操作系统I/O过程中，需要把数据从用户态拷贝到内核态，然后再输出到I/O设备，所以从java堆内存输出到I/O设备需经过两次拷贝，而direct memory在native 堆上，所以只需经过一次拷贝。
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