sun.misc.Cleaner

java NIO包是通过sun.misc.Cleaner和PhantomReference来实现堆外内存的自动释放的。现在来学习下Cleaner的使用

 

sun.misc.Cleaner是JDK内部提供的用来释放堆外内存的API

 

package com.cq.myproject.cleaner;

import java.lang.reflect.Field;

import sun.misc.Unsafe;

public class FreeMemoryTask implements Runnable {
	
	private long address;
	
	public FreeMemoryTask(long address)
	{
		this.address = address;
	}

	public void run() {
		System.out.println("running freeMemoryTask");
		if(address == 0)
		{
			System.out.println("already released");
		}
		else {
			getUnsafeInstance().freeMemory(address);
			System.out.println("released");
		}
	}

	public static Unsafe getUnsafeInstance() {
		try {
			Field theUnsafeInstance = Unsafe.class.getDeclaredField("theUnsafe");
			theUnsafeInstance.setAccessible(true);
			return (Unsafe) theUnsafeInstance.get(Unsafe.class);
		} catch (Exception e) {
			return null;
		} 
	}
}

 这个实现Runnable接口的类，功能就是释放堆外内存。这是我们必须要做的是，JVM没办法帮我们做（这里的Unsafe对象要手动import）

 

package com.cq.myproject.cleaner;
import sun.misc.Cleaner;
public class ObjectInHeapUseCleaner {
	private static long address = 0;
	
	public ObjectInHeapUseCleaner(){
		address = FreeMemoryTask.getUnsafeInstance().allocateMemory(2*1024*1024);
	}
	
	public static void main(String[] args) {
		while(true)
		{
			System.gc();
			ObjectInHeapUseCleaner heap = new ObjectInHeapUseCleaner();
			Cleaner.create(heap,new FreeMemoryTask(address));
		}
	}
}

 

运行这段代码，程序正常运行不会出现OOM（看任务管理器内存使用率不动，cpu使用率跳高）

 

Cleaner.create()需要两个参数：需要监控的内存对象和程序释放资源的回调。当JVM进行GC时，如果发现我们监控的对象，不存在强引用（Cleaner对象对他的引用是幽灵引用），就会调用第二个参数的run()方法逻辑，执行完run()方法时（此时已经释放了堆外内存），JVM会自动释放堆内存中我们监控的对象