Java对象销毁和finalize方法怎么使用-java教程-PHP中文网

对象的销毁

在C++中析构方法用于释放资源并且销毁对象本身。

在Java中，由于GC的存在，我们不需要手动回收内存，这大大减少了工作量，也提高了程序的安全性。但是Java也确实存在一个类似于C++中析构的函数。

finalize方法

重载该方法，用于在类被GC回收的时候执行一些操作。

下面是一个类实现finalize的示例。

Aoo类具有一个int 一个String属性，重载了toString并且在构造其中打印这个对象及其创建时间，在finalize中打印这个对象及调用时间。

Aoo类

public class Aoo {
       private int id;
       private String name;

       public Aoo(){
              this(0, null);
       }

       public Aoo(int id, String name){
              this.id = id;
              this.name = name;
              System.out.println(this.toString() + " now create:" + System.currentTimeMillis());
       }
        /*
        * 省略get/set/toString
        */
       protected void finalize() throws Throwable{
              super.finalize();
              System.out.println(this.toString() + "now finalize:" + System.currentTimeMillis());
       }
}

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首先，一个简单的测试

main方法

public class FinalizeTest {
       public static void main(String[] args) throws Exception {
              Aoo a = new Aoo(1, "a");
              a = null;
              System.gc()
              Thread.sleep(2000);
              System.exit(0);
       }
}

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打印结果：

id:1 name:a now create:1497547723036
id:1 name:anow finalize:1497547724059

GC对对象的回收

这里手动调用了GC来清理内存，而如果将其注释掉System.gc();，打印结果是这样的：

id:1 name:a now create:1497547846923

也就是说，在没有特意调用GC的情况下，finalize方法根本没有被调用，也就是说这个对象根本没有被主动回收。

和想象中的不同，GC的运行方式是惰性的，也就是说，在内存没有一处的情况下，GC不会去主动回收对象，为了验证这个想法，我创建了一个线程，用于不断的消耗内存,并且不主动调用GC。

ThreadA类

public class ThreadA implements Runnable{
       public void run() {
              List<Integer> list = new ArrayList<Integer>();
              int i = 0;
              while(true){
                     list.add(i);
                     i++;
              }
       }
}

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main方法

public class FinalizeTest {
       public static void main(String[] args) throws Exception {
              Aoo a = new Aoo(1, "a");
              a = null;
              ThreadA ta = new ThreadA();
              Thread t = new Thread(ta);
              t.start();
              Thread.sleep(2000);
              System.exit(0);
       }
}

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打印结果：

id:1 name:a now create:1497548135268
id:1 name:anow finalize:1497548135386

这一次尽管没有手动调用GC,但是finalize方法仍然运行了，也就是说，只有在内存被消耗、需要GC出面清理内存的时候，GC才会运行。

这样的finalize方法确实不靠谱，连能不能被调用都不一定，更不用说完成什么特定的操作了，如果需要关流等回收资源，不如手动调用一个方法，或者在final块中统一释放资源。

在finalize方法中，是否重新给自己指定一个引用来避免被GC回收？

尝试在finalize方法中重新引用来让GC无法回收

修改后的Aoo如下

public class Aoo {
       public static Aoo SAVE = null;
       private int id;
       private String name;

       public Aoo(){
              this(0, null);
       }

       public Aoo(int id, String name){
              this.id = id;
              this.name = name;
              System.out.println(this.toString() + " now create:" + System.currentTimeMillis());
       }  
       /*
        * 省略get/set/toString
        */
       protected void finalize() throws Throwable{
              super.finalize();
              System.out.println(this.toString() + "now finalize:" + System.currentTimeMillis());
              SAVE = this;
       }
}

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main方法

public class FinalizeTest {
       public static void main(String[] args) throws Exception {
              Aoo.SAVE = new Aoo(1, "a");
              Aoo.SAVE = null;
              System.gc();
              Thread.sleep(500);
              System.out.println(Aoo.SAVE == null? "a is dead" : "a is alive" );              
              System.exit(0);             
       }
}

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打印结果：

id:1 name:a now create:1497551409195
id:1 name:anow finalize:1497551409201
a is alive

这里看出，Aoo.SAVE对象确实“复活了”，不过这样的操作是有限制的，如果故技重施不会再一次“复活”该对象。

main方法

public class FinalizeTest {
       public static void main(String[] args) throws Exception {
              Aoo.SAVE = new Aoo(1, "a");
              Aoo.SAVE = null;
              System.gc();
              Thread.sleep(500);
              System.out.println(Aoo.SAVE == null? "a is dead" : "a is alive" );

              Aoo.SAVE = null;
              System.gc();
              Thread.sleep(500);
              System.out.println(Aoo.SAVE == null? "a is dead" : "a is alive" );

              System.exit(0);          
       }
}

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打印结果：

id:1 name:a now create:1497551587715
id:1 name:anow finalize:1497551587721
a is alive
a is dead

这里注意到，两次的操作是相同的，而finalize方法只会被系统调用一次。

如果finalze方法中出现死循环会发生什么？

Aoo类

public class Aoo {
       private int id;
       private String name;

       public Aoo(){
              this(0, null);
       }

       public Aoo(int id, String name){
              this.id = id;
              this.name = name;
              System.out.println(this.toString() + " now create:" + System.currentTimeMillis());
       }
       /*
        * 省略get/set/toString
        */
       protected void finalize() throws Throwable{
              super.finalize();
              while(true){
                     System.out.println(this.toString() + "now finalize:" + System.currentTimeMillis());
                     Thread.sleep(100);
              }
       }
}

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main方法

public class FinalizeTest {
       public static void main(String[] args) throws Exception {
              Aoo a1 = new Aoo(1 , "a1");
              Aoo a2 = new Aoo(2 , "a2");
              a1 = null;
              a2 = null;
              ThreadA ta = new ThreadA();
              Thread t = new Thread(ta);
              t.start();
              Thread.sleep(5000);

              System.exit(0);
       }
}

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打印结果：

id:1 name:a1 now create:1497552024252
id:2 name:a2 now create:1497552024252
id:1 name:a1now finalize:1497552024373
id:1 name:a1now finalize:1497552024503
id:1 name:a1now finalize:1497552026848
id:1 name:a1now finalize:1497552028960
id:1 name:a1now finalize:1497552032363

结果是随机的，有时候是执行的a1的finalize，有的时候执行的是a2的。

这个结果说明了两点：

1.finalze方法在的线程优先级很低，时间间隔相当的不确定并且明显大于100毫秒。

2.这个死循环导致了别的对象的finalize方法无法执行。

如果对象的创建出现这种死循环，会不会导致对象无法销毁进而导致内存溢出？

我们大量创建Aoo对象，并且等待GC自己回收内存。

为了直观的观看finalize方法的调用情况，删除掉了Aoo对象初始化的时候的打印代码。

main方法

public class FinalizeTest {
       public static void main(String[] args) throws Exception {
              int i = 1;
              while(true){
                     Aoo a = new Aoo(i , "a" + i);
                     i++;
              }
       }
}

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让程序执行了约两分钟，然后手动终止，查看输出

1497554225913
id:269614 name:a269614now finalize:1497554226151
id:269614 name:a269614now finalize:1497554227635
id:269614 name:a269614now finalize:1497554227735
id:269614 name:a269614now finalize:1497554227836
id:269614 name:a269614now finalize:1497554229586
id:269614 name:a269614now finalize:1497554229686
id:269614 name:a269614now finalize:1497554229951
id:269614 name:a269614now finalize:1497554230051
id:269614 name:a269614now finalize:1497554230152
id:269614 name:a269614now finalize:1497554233699
id:269614 name:a269614now finalize:1497554233800
id:269614 name:a269614now finalize:1497554233900
id:269614 name:a269614now finalize:1497554234308
id:269614 name:a269614now finalize:1497554234408
id:269614 name:a269614now finalize:1497554234508
id:269614 name:a269614now finalize:1497554235053
id:269614 name:a269614now finalize:1497554235153
id:269614 name:a269614now finalize:1497554235253
id:269614 name:a269614now finalize:1497554235823
id:269614 name:a269614now finalize:1497554235923
id:269614 name:a269614now finalize:1497554236023
id:269614 name:a269614now finalize:1497554240324
id:269614 name:a269614now finalize:1497554240424
id:269614 name:a269614now finalize:1497554240525
id:269614 name:a269614now finalize:1497554241146
id:269614 name:a269614now finalize:1497554241247
id:269614 name:a269614now finalize:1497554241347
id:269614 name:a269614now finalize:1497554241448
id:269614 name:a269614now finalize:1497554242020
id:269614 name:a269614now finalize:1497554242120
id:269614 name:a269614now finalize:1497554242220
id:269614 name:a269614now finalize:1497554242321
id:269614 name:a269614now finalize:1497554242421
id:269614 name:a269614now finalize:1497554242521
id:269614 name:a269614now finalize:1497554248367
id:269614 name:a269614now finalize:1497554248467
id:269614 name:a269614now finalize:1497554248567
id:269614 name:a269614now finalize:1497554248667
id:269614 name:a269614now finalize:1497554249534
id:269614 name:a269614now finalize:1497554249634
id:269614 name:a269614now finalize:1497554249734
id:269614 name:a269614now finalize:1497554249835
id:269614 name:a269614now finalize:1497554255954
id:269614 name:a269614now finalize:1497554256055
id:269614 name:a269614now finalize:1497554256155
id:269614 name:a269614now finalize:1497554256255
id:269614 name:a269614now finalize:1497554256356
id:269614 name:a269614now finalize:1497554257285
id:269614 name:a269614now finalize:1497554257386
id:269614 name:a269614now finalize:1497554257486
id:269614 name:a269614now finalize:1497554257586
id:269614 name:a269614now finalize:1497554257686
id:269614 name:a269614now finalize:1497554268652
id:269614 name:a269614now finalize:1497554268753
id:269614 name:a269614now finalize:1497554268853
id:269614 name:a269614now finalize:1497554268953
id:269614 name:a269614now finalize:1497554269054
id:269614 name:a269614now finalize:1497554269154
id:269614 name:a269614now finalize:1497554277474
id:269614 name:a269614now finalize:1497554292852
id:269614 name:a269614now finalize:1497554301062

可以发现两个情况：

1.只有一个对象的finalize方法被执行了，也就是说这个死循环的finalize方法阻止了其他对象执行finalize方法

2.程序执行很快的一段时间后，finalize方法就开始执行，但是随着内存消耗的不断增加，finalize方法被执行的次数也就越来越少。

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