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 | BigMemory stores “big” amounts of data in machine memory for ultra-fast access | |
| 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 | 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
参考资料
[1].
[2]. [3].
import java.lang.reflect.Method;import java.nio.ByteBuffer;import java.security.AccessController;import java.security.PrivilegedAction;/** * {@link DirectByteBufferCleaner} * * @author zhongl * @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