DirectByteBuffer

最新推荐文章于 2022-10-23 10:03:15 发布

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最新推荐文章于 2022-10-23 10:03:15 发布

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本文详细介绍了DirectBuffer的概念及其在Java NIO中的实现方式，包括不同模式下MappedByteBuffer的应用场景及注意事项。此外，还提供了如何手动清理DirectBuffer以避免内存泄漏的方法。

DirectBuffer 通过免去中间交换的内存拷贝, 提升IO处理速度;也就是常说的zero-copy

ByteBuffer
- HeapByteBuffer
- MappedByteBuffer
  - MappedByteBuffer.map(int mode,long position,long size);

`READ_ONLY`	只读	试图修改得到的缓冲区将导致抛出 ReadOnlyBufferException
`READ_WRITE`	读/写	对得到的缓冲区的更改最终将传播到文件；该更改对映射到同一文件的其他程序不一定是可见的
`PRIVATE`	专用	对得到的缓冲区的更改不会传播到文件，并且该更改对映射到同一文件的其他程序也不是可见的；相反，会创建缓冲区已修改部分的专用副本

分配	`java.nio.ByteBuffer.allocateDirect`
释放	`sun.nio.ch.DirectBuffer.cleaner().clean()`

可以通过参数 -XX:MaxDirectMemorySize= 设置其可使用的最大内存大小，当未设置时默认同 -Xmx 的大小。

各种应用

`BigMemory`	http://terracotta.org/products/bigmemory	BigMemory stores “big” amounts of data in machine memory for ultra-fast access
`DirectMemory`	http://incubator.apache.org/directmemory/	Apache DirectMemory is a multi layered cache implementation featuring off-heap memory management (a-la BigMemory) to enable efficient handling of a large number of java objects without affecting jvm garbage collection performance
`HugeCollections`	https://code.google.com/p/vanilla-java/wiki/HugeCollections	If you want to efficiently store large collections of data in memory. This library can dramatically reduce Full GC times and reduce memory consumption as well

OutOfMemoryError

和 DirectBuffer 相关的内存泄漏参看Study_Java_HotSpot_OOME

参考资料

[1]. http://blog.youkuaiyun.com/fancyerii/article/details/7655451
[2]. http://lixjluck.iteye.com/blog/1130455
[3]. http://www.zhurouyoudu.com/index.php/archives/470/

import java.lang.reflect.Method;
import java.nio.ByteBuffer;
import java.security.AccessController;
import java.security.PrivilegedAction;

/**
 * {@link DirectByteBufferCleaner}
 *
 * @author <a href=mailto:zhong.lunfu@gmail.com>zhongl</a>
 * @created 2011-1-14
 */
public final class DirectByteBufferCleaner {
  private DirectByteBufferCleaner() {}

  public static void clean(final ByteBuffer byteBuffer) {
    if (!byteBuffer.isDirect()) return;
    try {
      Object cleaner = invoke(byteBuffer, "cleaner");
      invoke(cleaner, "clean");
    } catch (Exception e) { /* ignore */ }
  }

  private static Object invoke(final Object target, String methodName) throws Exception {
    final Method method = target.getClass().getMethod(methodName);
    return AccessController.doPrivileged(new PrivilegedAction<Object>() {
      @Override
      public Object run() {
        try {
          return method.invoke(target);
        } catch (Exception e) {
          throw new RuntimeException(e);
        }
      }
    });
  }

}