java的几种引用类型

简单看了一下java的几种引用类型,先贴着,有时间来整理.

import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;


public class ReferenceTest {
	
	public static void main(String[] args) throws Throwable {
//		strong();
		soft();
//		weak();
	}
	
	/**
	 * 强引用
	 * (最常见的引用,如果一个对象具有强引用, VM宁愿抛出 OutOfMemoryError 都不会回收它)
	 */
	public static void strong(){
		System.out.println("MAX Mem\t:" + (Runtime.getRuntime().maxMemory()/1024/1024));
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		String[] strs = new String[9999999]; //存放String数组对象的强引用
		System.out.println("=======================");
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		String[] strs1 = new String[9999999]; //存放String数组对象的强引用
	}

	/**
	 * 软引用
	 * (不保证对象一定会被回收:不管是否显式gc,内存不足时才会回收)
	 */
	public static void soft(){
		System.out.println("MAX Mem\t:" + (Runtime.getRuntime().maxMemory()/1024/1024));
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		String[] strs = new String[9999999]; //存放String对象的强引用
		System.out.println("=======================");
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		SoftReference<String[]> sr = new SoftReference<String[]>(strs);//一个软引用指向strs
		strs = null;//释放strs的强引用
		System.out.println(sr.get());//回收以后通过软引用访问strs(没有被回收)
		System.gc();
		System.out.println(sr.get());//回收以后通过软引用访问strs(没有被回收)
		System.out.println("=======================");
		String[] strs1 = new String[9999999]; //存放String对象的强引用
		System.out.println(sr.get());//回收以后通过软引用访问strs(已经被回收)
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
	}
	
	/**
	 * 弱引用
	 * (对象一定会被回收:调用gc后马上回收)
	 */
	public static void weak(){
		System.out.println("MAX Mem\t:" + (Runtime.getRuntime().maxMemory()/1024/1024));
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		String[] strs = new String[9999999]; //存放String对象的强引用
		System.out.println("=======================");
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		WeakReference<String[]> sr = new WeakReference<String[]>(strs);//一个弱引用指向strs
		strs = null;//释放strs的强引用
		System.out.println("GC前: \t" + sr.get());//回收以后通过弱引用访问strs(GC前没有被回收)
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		System.gc();
		System.out.println("GC后: \t" + sr.get());//回收以后通过弱引用访问strs(GC后已经被回收)
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
		System.out.println("=======================");
		String[] strs1 = new String[9999999]; //存放String对象的强引用
		System.out.println("GC后: \t" + sr.get());//回收以后通过弱引用访问strs(GC后已经被回收)
		System.out.println("FREE Mem\t:" + (Runtime.getRuntime().freeMemory()/1024/1024));
	}
	
}