package com.sgcc.test;
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;

public class test {
	
	public static void main(String[] args) throws InterruptedException {
 

		//強引用
    	Object obj = new Object();
    	Object objr;
    	//可直接通過obj取得對應的物件 如
    	if(obj.equals(objr = new Object()))System.out.println(123);
    	obj = null;
    	Runtime.getRuntime().gc();
    	System.out.println(objr);
    	
    	//軟引用
    	Object obj1 = new Object();
    	SoftReference<Object>
 
 sf = new SoftReference<Object>(obj1);
    	obj1 = null;
    	Runtime.getRuntime().gc();
    	System.out.println(sf.get());//有時候會返回null
    	
    	//弱引用
    	Object obj2 = new Object();
    	WeakReference<Object> wf = new WeakReference<Object>(obj2);
    	obj2 = null;
    	Runtime.getRuntime
 
().gc();
    	System.out.println(wf.get());
    	
    	//虛引用
    	Object obj3 = new Object();
    	final ReferenceQueue q = new ReferenceQueue();
    	PhantomReference<Object> pf = new PhantomReference<Object>(obj3, q);
    	Thread t = new Thread(){
    		@Override
    		public void run() {
    			// TODO Auto-generated method stub
    			try {
    				Reference reference = q.remove();
					System.out.println(reference + " event fired.");
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
    			super.run();
    		}
    	};
    	
    	t.setDaemon(true);
		t.start();
    	obj3=null;
    	//Runtime.getRuntime().gc();
    	System.gc();
    	try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
    	System.out.println("pf.get: "+pf.get() +" isEnqueued"+ pf.isEnqueued());//返回是否從記憶體中已經刪除
	}

}

//gc回收後的自救
package com.sgcc.test;

public class FinalizeEscapeGC {
	
	public String name;
    public static FinalizeEscapeGC SAVE_HOOK = null;

    public FinalizeEscapeGC(String name) {
        this.name = name;
    }

    public void isAlive() {
        System.out.println("yes, i am still alive :)");
    }
    
    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("finalize method executed!");
        System.out.println(this);
        FinalizeEscapeGC.SAVE_HOOK = this;
    }

    @Override
    public String toString() {
        return name;
    }

    public static void main(String[] args) throws InterruptedException {
        SAVE_HOOK = new FinalizeEscapeGC("leesf");
        System.out.println(SAVE_HOOK);
        // 物件第一次拯救自己
        SAVE_HOOK = null;
        System.out.println(SAVE_HOOK);
        System.gc();
        // 因為finalize方法優先順序很低，所以暫停0.5秒以等待它
        Thread.sleep(500);
        if (SAVE_HOOK != null) {
            SAVE_HOOK.isAlive();
        } else {
            System.out.println("no, i am dead : (");
        }

        // 下面這段程式碼與上面的完全相同,但是這一次自救卻失敗了
        // 一個物件的finalize方法只會被呼叫一次
        SAVE_HOOK = null;
        System.gc();
        // 因為finalize方法優先順序很低，所以暫停0.5秒以等待它
        Thread.sleep(500);
        if (SAVE_HOOK != null) {
            SAVE_HOOK.isAlive();
        } else {
            System.out.println("no, i am dead : (");
        }
    }
}

參考文章地址：https://www.cnblogs.com/yw-ah/p/5830458.html