java的finalize方法

本文主要是介绍java的finalize方法，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

说明

不建议在实际代码中使用

当一个对象被可行性分析判断为不可达对象的时候，会被第一次标记，判断finalize方法是否重写，或者已经执行，如果没有，那么该对象会被分配到一个F-Queue队列中。虚拟机会自动建立一个低调度优先级的Finalizer线程去执行。当再次标记为不可达对象的时候，不再执行。

只会执行一次
允许该对象在方法内被再次调用（复活）
finalize方法内的任何异常都会忽略

《Java并发编程实战》（Java Concurrency in Practice）

当不再需要内存资源时，可以通过垃圾收集器来回收它们，但对于其他一些资源，例如文件句柄或套接字句柄，当不再需要它们时，必须显式地交给操作系统。
为了实现这个功能垃圾收集对那些定义了 finalize 方法的对象会进行特殊处理：在回收器释放它们后，调用它们的 finalize 方法，从而保证一些持久化的资源被释放。
大多数情况下，通过使用 finally 代码块和显式的 close 方法，能够比使用终结器更好的管理资源。
唯一的例外情况在于：当需要管理对象，并且该对象持有的资源是通过本地方法获得的。其他情况都应该尽量避免使用。

源码注释

    /*** Called by the garbage collector on an object when garbage collection* determines that there are no more references to the object.* A subclass overrides the {@code finalize} method to dispose of* system resources or to perform other cleanup.* <p>* The general contract of {@code finalize} is that it is invoked* if and when the Java&trade; virtual* machine has determined that there is no longer any* means by which this object can be accessed by any thread that has* not yet died, except as a result of an action taken by the* finalization of some other object or class which is ready to be* finalized. The {@code finalize} method may take any action, including* making this object available again to other threads; the usual purpose* of {@code finalize}, however, is to perform cleanup actions before* the object is irrevocably discarded. For example, the finalize method* for an object that represents an input/output connection might perform* explicit I/O transactions to break the connection before the object is* permanently discarded.* <p>* The {@code finalize} method of class {@code Object} performs no* special action; it simply returns normally. Subclasses of* {@code Object} may override this definition.* <p>* The Java programming language does not guarantee which thread will* invoke the {@code finalize} method for any given object. It is* guaranteed, however, that the thread that invokes finalize will not* be holding any user-visible synchronization locks when finalize is* invoked. If an uncaught exception is thrown by the finalize method,* the exception is ignored and finalization of that object terminates.* <p>* After the {@code finalize} method has been invoked for an object, no* further action is taken until the Java virtual machine has again* determined that there is no longer any means by which this object can* be accessed by any thread that has not yet died, including possible* actions by other objects or classes which are ready to be finalized,* at which point the object may be discarded.* <p>* The {@code finalize} method is never invoked more than once by a Java* virtual machine for any given object.* <p>* Any exception thrown by the {@code finalize} method causes* the finalization of this object to be halted, but is otherwise* ignored.** @throws Throwable the {@code Exception} raised by this method* @see java.lang.ref.WeakReference* @see java.lang.ref.PhantomReference* @jls 12.6 Finalization of Class Instances*/

java测试类

package com.hyl.learnerJVM.gc;/*** finalize执行测试* <p>** @author Hyl* @version V 0.1* @since 0.1 2020-07-03 04:57*/
public class FinalizeMethod {public static FinalizeMethod Fin = null;@Overrideprotected void finalize() throws Throwable {super.finalize();System.out.println("finalize run !");// 通过finalize救赎一次，仅能一次FinalizeMethod.Fin = this;// 任何异常会被忽略掉throw new RuntimeException("我就是想试试看");}public void isAlive() {System.out.println("I am alive");}public static void main(String[] args) throws Exception {Fin = new FinalizeMethod();// 第一次自我毁灭Fin = null;System.gc();Thread.sleep(500);if (Fin != null) {Fin.isAlive();} else {System.out.println("I am dead!");}// 第二次自我毁灭Fin = null;System.gc();Thread.sleep(500);if (Fin != null) {Fin.isAlive();} else {System.out.println("I am dead!");}}
}