Java对象销毁和finalize方法

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());       }}

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);       }}

打印结果：

id:1 name:a now create:1497547723036id: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++;              }       }}

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);       }}

打印结果：

id:1 name:a now create:1497548135268id: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;       }}

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);                    }}

打印结果：

id:1 name:a now create:1497551409195id:1 name:anow finalize:1497551409201a 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);                 }}

打印结果：

id:1 name:a now create:1497551587715id:1 name:anow finalize:1497551587721a is alivea 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);              }       }}

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);       }}

打印结果：

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

让程序执行了约两分钟，然后手动终止，查看输出

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

总结

至此，我尝试了finalize方法的一些用法和特殊情况。可以看出，GC调用finalize方法存在巨大的不确定性，确实很不靠谱，不过通过这个方法，了解了一些关于GC的知识，也让我明白，虽然Java语言虽然具有高度的一致性等特点使之很容易上手，但是要做到对Java的精通，路还很远呢~~