“对于任何给定对象，JVM 永远不会多次调用 finalize 方法”的含义。在物体复活的情况下？

Question

根据 Java 文档：对于任何给定对象，Java 虚拟机永远不会多次调用 finalize 方法。上述政策可以帮助使对象不朽吗？如果我从 finalize() 方法中一次复活一个对象，那么该对象是否会成为不朽的，因为 JVM 不能第二次在该对象上调用 finalize() 方法？或者如果该对象再次符合 GC 条件（假设指向复活对象的另一个引用符合 GC 条件），那么下次 JVM 将删除该对象而不调用该对象的 finalize() 方法？我试图通过一些示例代码来验证它并假设第二个是正确的，但需要确认我的理解。

Answer 1

根据 ernest_k，它是第 2 个。该对象将被垃圾收集，而无需调用其 finalize() 方法。

添加我的示例代码来完成这个答案，我创建它来验证。

import java.util.HashSet;

import java.util.Set;

class Immortal {

    // making it public and non-fianl to modify from outside

    public static Set<Immortal> immortals = new HashSet<>();

    @Override

    protected void finalize() throws Throwable {

        System.out.println("Running finalize for " + this);

        immortals.add(this); // Resurrect the object by creating a new reference

    }

}

public class ObjectResurrection {

    public static void callGC() {

        // call garbage collection

        System.gc();

        // wait for some time to give chance to run garbage collection thread

        try {

            Thread.sleep(10 * 1000);

        } catch (InterruptedException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

    }

    public static void main(String[] args) {

        Immortal immortal1 = new Immortal();

        Immortal immortal2 = new Immortal();

        // print objects when they are alive.

        System.out.println("immortal1 = " + immortal1);

        System.out.println("immortal2 = " + immortal2);

        // print immortal set

        System.out.println("immortal set = " + Immortal.immortals);

        // make all objects garbage collectable

        immortal1 = null;

        immortal2 = null;

        callGC();

        // now objects will be null

        System.out.println("immortal1 = " + immortal1);

        System.out.println("immortal2 = " + immortal2);

        // but stays in immortal set

        System.out.println("immortal set = " + Immortal.immortals);

        // remove all objects from immortal set, and make them again eligible for GC

        Immortal.immortals.clear();

        callGC();

        // again print the immortal set

        // this time set will be empty, and those 2 objects will be destroyed by Garbage

        // Collection,

        // but this time they will not call finalize, because it is already called

        // you can notice no output from finalize method in the output this time

        System.out.println("immortal set = " + Immortal.immortals);

    }

}

上述程序的输出（在我的机器上）：

immortal1 = effective.java.item7.finalizer6.Immortal@70dea4e

immortal2 = effective.java.item7.finalizer6.Immortal@5c647e05

immortal set = []

Running finalize for effective.java.item7.finalizer6.Immortal@5c647e05

Running finalize for effective.java.item7.finalizer6.Immortal@70dea4e

immortal1 = null

immortal2 = null

immortal set = [effective.java.item7.finalizer6.Immortal@5c647e05, effective.java.item7.finalizer6.Immortal@70dea4e]

immortal set = []