对象复活时"The finalize method is never invoked more than once by a JVM for any given object."的含义？

Question

根据 Java 文档：

对于任何给定的对象，Java 虚拟机永远不会多次调用 finalize 方法。

以上的政策能否帮助使一个对象永生？如果我从 finalize() 方法复活一个对象一次，那么该对象是否会永生，因为 JVM 无法对该对象第二次调用 finalize() 方法？

或

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？

我已经尝试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。对象将在不调用其 finalize() 方法的情况下被垃圾收集。

添加我的示例代码来完成这个答案，我创建它是为了验证。

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);
    }

}

以上程序的输出（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 = []

对象复活时"The finalize method is never invoked more than once by a JVM for any given object."的含义？

Meaning of "The finalize method is never invoked more than once by a JVM for any given object." in case of Object Resurrection?

java

finalizer