Sometimes when it is destroyed, an object must make an action. For example, if an object has a non-java resource such as a file handle or a font, you can verify that these resources are released before destroying an object. To manage such situations, java offers a mechanism called "finalizing". By finalizing it, you can define specific actions that occur when an object is about to be removed from the garbage collector. To add a finalizer to a class simply define the finalize() method. Java execution time calls this method whenever it is about to delete an object of that class. Within the finalize method() you specify actions to be performed before destroying an object. The garbage collector is periodically searched for objects that no longer refer to any running state or indirectly any other object with reference. Before an asset is released, the Java runtime calls the finalize() method on the object. The finalize() method has the following general form:

protected void finalize(){
    // This is where the finalization code is entered
}

使用protected关键字，finalize()类之外的代码可以阻止对finalize()的访问。 务必要理解finalize()是在垃圾收集之前调用的。例如，当对象离开作用域时，它不会被调用。这意味着您无法知道finalize()何时或是否将执行。因此，程序必须提供其他方法来释放系统资源或对象使用的其他资源。不应该依赖finalize()来实现程序的正常运行。

finalize方法在对象即将被垃圾收集时被调用。这可以在它有资格进行垃圾收集之后的任何时间进行。

注意，完全有可能一个对象永远不会被垃圾收集(因此finalize永远不会被调用)。当对象永远不符合gc条件(因为在JVM的整个生命周期内都可以访问它)，或者在对象符合条件到JVM停止运行之间没有实际运行垃圾收集时(这种情况经常发生在简单的测试程序中)，就会发生这种情况。

有一些方法可以告诉JVM在尚未调用的对象上运行finalize，但使用这些方法也不是一个好主意(该方法的保证也不是很强)。

如果您依赖finalize来实现应用程序的正确操作，那么您就做错了一些事情。finalize只能用于清理(通常是非java)资源。这正是因为JVM不能保证finalize在任何对象上都被调用。

Java allows objects to implement a method called finalize() that might get called. finalize() method gets called if the garbage collector tries to collect the object. If the garbage collector doesn't run, the method doesn't get called. If the garbage collector fails to collect the object and tries to run it again, the method doesn't get called in the second time. In practice, you are highly unlikely to use it in real projects. Just keep in mind that it might not get called and that it definitely won't be called twice. The finalize() method could run zero or one time. In the following code, finalize() method produces no output when we run it since the program exits before there is any need to run the garbage collector.

源

由于JVM调用finalize()方法存在不确定性(不确定被重写的finalize()是否会被执行)，为了研究目的，观察finalize()调用时发生的情况的更好方法是通过命令System.gc()强制JVM调用垃圾收集。

具体来说，finalize()在对象不再使用时被调用。但是当我们试图通过创建新对象来调用它时，它的调用是不确定的。因此，为了确定起见，我们创建了一个空对象c，它显然没有未来的用途，因此我们看到对象c的finalize调用。

例子

class Car {

    int maxspeed;

    Car() {
        maxspeed = 70;
    }

    protected void finalize() {

    // Originally finalize method does nothing, but here we override finalize() saying it to print some stmt
    // Calling of finalize is uncertain. Difficult to observe so we force JVM to call it by System.gc(); GarbageCollection

        System.out.println("Called finalize method in class Car...");
    }
}

class Bike {

    int maxspeed;

    Bike() {
        maxspeed = 50;
    }

    protected void finalize() {
        System.out.println("Called finalize method in class Bike...");
    }
}

class Example {

    public static void main(String args[]) {
        Car c = new Car();
        c = null;    // if c weren`t null JVM wouldn't be certain it's cleared or not, null means has no future use or no longer in use hence clears it
        Bike b = new Bike();
        System.gc();    // should clear c, but not b
        for (b.maxspeed = 1; b.maxspeed <= 70; b.maxspeed++) {
            System.out.print("\t" + b.maxspeed);
            if (b.maxspeed > 50) {
                System.out.println("Over Speed. Pls slow down.");
            }
        }
    }
}

输出

    Called finalize method in class Car...
            1       2       3       4       5       6       7       8       9
    10      11      12      13      14      15      16      17      18      19
    20      21      22      23      24      25      26      27      28      29
    30      31      32      33      34      35      36      37      38      39
    40      41      42      43      44      45      46      47      48      49
    50      51Over Speed. Pls slow down.
            52Over Speed. Pls slow down.
            53Over Speed. Pls slow down.
            54Over Speed. Pls slow down.
            55Over Speed. Pls slow down.
            56Over Speed. Pls slow down.
            57Over Speed. Pls slow down.
            58Over Speed. Pls slow down. 
            59Over Speed. Pls slow down.
            60Over Speed. Pls slow down.
            61Over Speed. Pls slow down.
            62Over Speed. Pls slow down.
            63Over Speed. Pls slow down.
            64Over Speed. Pls slow down.
            65Over Speed. Pls slow down.
            66Over Speed. Pls slow down.
            67Over Speed. Pls slow down.
            68Over Speed. Pls slow down.
            69Over Speed. Pls slow down.
            70Over Speed. Pls slow down.

注意:即使打印到70，并且在此之后对象b在程序中没有被使用，也不确定b是否被JVM清除，因为“调用的finalize方法在类Bike…”没有打印。

Finalize将打印出类创建的计数。

protected void finalize() throws Throwable {
    System.out.println("Run F" );
    if ( checkedOut)
        System.out.println("Error: Checked out");
        System.out.println("Class Create Count: " + classCreate);
}

main

while ( true) {
    Book novel=new Book(true);
    //System.out.println(novel.checkedOut);
    //Runtime.getRuntime().runFinalization();
    novel.checkIn();
    new Book(true);
    //System.runFinalization();
    System.gc();

如你所见。下面的输出显示了类计数为36时第一次执行的gc。

C:\javaCode\firstClass>java TerminationCondition
Run F
Error: Checked out
Class Create Count: 36
Run F
Error: Checked out
Class Create Count: 48
Run F

Sometimes when it is destroyed, an object must make an action. For example, if an object has a non-java resource such as a file handle or a font, you can verify that these resources are released before destroying an object. To manage such situations, java offers a mechanism called "finalizing". By finalizing it, you can define specific actions that occur when an object is about to be removed from the garbage collector. To add a finalizer to a class simply define the finalize() method. Java execution time calls this method whenever it is about to delete an object of that class. Within the finalize method() you specify actions to be performed before destroying an object. The garbage collector is periodically searched for objects that no longer refer to any running state or indirectly any other object with reference. Before an asset is released, the Java runtime calls the finalize() method on the object. The finalize() method has the following general form:

protected void finalize(){
    // This is where the finalization code is entered
}

使用protected关键字，finalize()类之外的代码可以阻止对finalize()的访问。 务必要理解finalize()是在垃圾收集之前调用的。例如，当对象离开作用域时，它不会被调用。这意味着您无法知道finalize()何时或是否将执行。因此，程序必须提供其他方法来释放系统资源或对象使用的其他资源。不应该依赖finalize()来实现程序的正常运行。