Java直接內存原理

時間 2019-11-29

標籤 java 直接內存原理欄目 Java 简体版

原文原文鏈接

直接內存

上述對直接內存的描述來自《深刻理解Java虛擬機》，寫明瞭直接內存不在java堆內，而且java堆內存往外寫須要拷貝到native堆。

而後我們先寫個代碼看看直接內存分配在哪一個區域

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<ByteBuffer> byteBufferList = new ArrayList<>();
        while (true) {
            ByteBuffer byteBuffer = ByteBuffer.allocateDirect(10240);
            byteBufferList.add(byteBuffer);
            Thread.sleep(100);
        }
    }
}
複製代碼

watch命令觀察變化內存 html

我使用的是64位centos7，虛擬地址<00007fffffffffff 是用戶空間內存[1]。ByteBuffer.allocateDirect()分配的是用戶空間的匿名內存。

再來看一下ByteBuffer.allocateDirect(int capacity)

ByteBuffer.allocateDirect調用了Unsafe.allocateMemory(long var1)，該方法是個native方法 github.com/unofficial-…java

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
複製代碼

主要就是os::malloc(sz, mtInternal)方法，該方法封裝了C++的標準庫std::malloc() github.com/unofficial-…linux

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) {
/*省略代碼*/
  u_char* ptr;
  ptr = (u_char*)::malloc(alloc_size);
/*省略代碼*/
  // we do not track guard memory
  return MemTracker::record_malloc((address)ptr, size, memflags, stack, level);
}
複製代碼

(u_char*)::malloc(alloc_size)就是C++標準庫函數，傳入空間大小返回內存地址。git

那麼堆內存怎麼往外寫

openJDK sun.nio.ch.IOUtil.write()，java.nio.channels.SocketChannel#write(java.nio.ByteBuffer)等方法底層實現github

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);
        }
    }
複製代碼