jdk1.8 源码解读系列（一）——Object类

今天开始看jdk1.8的源码，源码阅读顺序是按照下面博客来的。
JDK源码阅读顺序

废话不多说，先上Object类的源码及简单注释。
新人第一次看源码，难免有错误之处，请各位多多包涵，若能指教几句，不胜感激！

/*
 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 */

package java.lang;

/**
 * Class {@code Object} is the root of the class hierarchy.
 * Every class has {@code Object} as a superclass. All objects,
 * including arrays, implement the methods of this class.
 *
 * @author  unascribed
 * @see     java.lang.Class
 * @since   JDK1.0
 */
public class Object {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    /**
     * Returns the runtime class of this {@code Object}. The returned
     * {@code Class} object is the object that is locked by {@code
     * static synchronized} methods of the represented class.
     *
     * 
The actual result type is {@code Class}
     * where {@code |X|} is the erasure of the static type of the
     * expression on which {@code getClass} is called. For
     * example, no cast is required in this code fragment:
     *
     * 

     * {@code Number n = 0;                             }

     * {@code Class c = n.getClass(); }
     * 
     *
     * @return The {@code Class} object that represents the runtime
     *         class of this object.
     * @jls 15.8.2 Class Literals
     */
    /**
     * 返回该对象的运行时类。该返回的Class对象是被静态同步方法锁定的对象。
     * @return
     */
    public final native Class getClass();

    /**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hash tables such as those provided by
     * {@link java.util.HashMap}.
     * 

     * The general contract of {@code hashCode} is:
     * 

     * 
Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the {@code hashCode} method
     *     must consistently return the same integer, provided no information
     *     used in {@code equals} comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * 
If two objects are equal according to the {@code equals(Object)}
     *     method, then calling the {@code hashCode} method on each of
     *     the two objects must produce the same integer result.
     * 
It is not required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the {@code hashCode} method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hash tables.
     * 
     * 

     * As much as is reasonably practical, the hashCode method defined by
     * class {@code Object} does return distinct integers for distinct
     * objects. (This is typically implemented by converting the internal
     * address of the object into an integer, but this implementation
     * technique is not required by the
     * Java™ programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    /**
     * 返回一个该对象的哈希值。该方法支持了像HashMap这一类的哈希表的实现。
     * 哈希值的通用约定是：
     * 1、在一个java应用执行期间，同一个对象无论调用该方法多少次，该方法返回的哈希值必须是一致的，当然，前提是该对象的equals比较方法用到的信息没有被修改。
     * 2、如果两个对象根据equals方法得到的结果是true，那么这两个对象分别调用该方法获取的哈希值必须是一致的。
     * 3、当两个对象根据equals方法得出不相等的情况是，不一定要求这两个对象各自获得的哈希值必须不同。然而，程序员应该意识到，为两个不相等的对象产生不同的哈希值，可能会提高哈希表的性能。
     * @return
     */
    public native int hashCode();

    /**
     * Indicates whether some other object is "equal to" this one.
     * 

     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * 

     * 
It is reflexive: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * 
It is symmetric: for any non-null reference values
     *     {@code x} and {@code y}, {@code x.equals(y)}
     *     should return {@code true} if and only if
     *     {@code y.equals(x)} returns {@code true}.
     * 
It is transitive: for any non-null reference values
     *     {@code x}, {@code y}, and {@code z}, if
     *     {@code x.equals(y)} returns {@code true} and
     *     {@code y.equals(z)} returns {@code true}, then
     *     {@code x.equals(z)} should return {@code true}.
     * 
It is consistent: for any non-null reference values
     *     {@code x} and {@code y}, multiple invocations of
     *     {@code x.equals(y)} consistently return {@code true}
     *     or consistently return {@code false}, provided no
     *     information used in {@code equals} comparisons on the
     *     objects is modified.
     * 
For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.
     * 
     * 

     * The {@code equals} method for class {@code Object} implements
     * the most discriminating possible equivalence relation on objects;
     * that is, for any non-null reference values {@code x} and
     * {@code y}, this method returns {@code true} if and only
     * if {@code x} and {@code y} refer to the same object
     * ({@code x == y} has the value {@code true}).
     * 

     * Note that it is generally necessary to override the {@code hashCode}
     * method whenever this method is overridden, so as to maintain the
     * general contract for the {@code hashCode} method, which states
     * that equal objects must have equal hash codes.
     *
     * @param   obj   the reference object with which to compare.
     * @return  {@code true} if this object is the same as the obj
     *          argument; {@code false} otherwise.
     * @see     #hashCode()
     * @see     java.util.HashMap
     */
    /**
     * 表示其他对象是否“等于”这个对象。
     * 该方法为非空对象实现了一个等价关系：
     * 1、自反性：如果有一个非空对象x，则x.equals(x)，一定返回true；
     * 2、对称性：如果有两个非空对象x,y,如果x.equals(y),返回true，那么y.equals(x)也一定返回true；
     * 3、传递性：如果有三个非空对象x,y,z,如果x.equals(y)返回true，且y.equals(z)返回true，那么x.equals(z)也一定返回true；
     * 4、一致性：如果有两个非空对象x,y，程序里多次调用equals()方法比较这两个对象，只要中间没有对这两个对象进行修改，那么调用该方法应该每次都返回一致信息。
     * 
     * 请注意，每当这个方法被覆盖时，通常都需要覆盖hashCode方法，以便维护hashCode方法的一般契约，该方法声明相等的对象必须具有相等的哈希值。
     * @param obj
     * @return
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }

    /**
     * Creates and returns a copy of this object.  The precise meaning
     * of "copy" may depend on the class of the object. The general
     * intent is that, for any object {@code x}, the expression:
     * 

     * 
     * x.clone() != x
     * will be true, and that the expression:
     * 

     * 
     * x.clone().getClass() == x.getClass()
     * will be {@code true}, but these are not absolute requirements.
     * While it is typically the case that:
     * 

     * 
     * x.clone().equals(x)
     * will be {@code true}, this is not an absolute requirement.
     * 

     * By convention, the returned object should be obtained by calling
     * {@code super.clone}.  If a class and all of its superclasses (except
     * {@code Object}) obey this convention, it will be the case that
     * {@code x.clone().getClass() == x.getClass()}.
     * 

     * 按照惯例，该方法返回的克隆对象应该是独立的。为了实现这种独立，在返回之前，可能需要通过super.clone()方法改变一个或多个该对象的字段。
     * 通常，这意味着，如果要复制的对象内部引用了一些可变对象，那么复制后返回的对象，对这些可变对象的引用应该是指向这些可变对象的副本。
     * By convention, the object returned by this method should be independent
     * of this object (which is being cloned).  To achieve this independence,
     * it may be necessary to modify one or more fields of the object returned
     * by {@code super.clone} before returning it.  Typically, this means
     * copying any mutable objects that comprise the internal "deep structure"
     * of the object being cloned and replacing the references to these
     * objects with references to the copies.  If a class contains only
     * primitive fields or references to immutable objects, then it is usually
     * the case that no fields in the object returned by {@code super.clone}
     * need to be modified.
     * 

     * 该方法执行了一个特殊的克隆操作。首先，如果该类没有实现Cloneable接口，就会抛出CloneNotSupportedException异常。
     * 要注意的是，所有数组类型都被认为已经实现了Cloneable接口，而且一个数组T[]调用该方法的返回类型是数组T[]，其中，T是任意的引用或原始数据类型。
     * 否则，该方法将创建该对象类的一个新实例，并使用该对象相应字段的内容初始化所有字段，就像通过赋值一样；这些字段的内容本身并不是克隆的。
     * 所以，该方法执行的是“浅拷贝”，而不是“深拷贝”。
     * The method {@code clone} for class {@code Object} performs a
     * specific cloning operation. First, if the class of this object does
     * not implement the interface {@code Cloneable}, then a
     * {@code CloneNotSupportedException} is thrown. Note that all arrays
     * are considered to implement the interface {@code Cloneable} and that
     * the return type of the {@code clone} method of an array type {@code T[]}
     * is {@code T[]} where T is any reference or primitive type.
     * Otherwise, this method creates a new instance of the class of this
     * object and initializes all its fields with exactly the contents of
     * the corresponding fields of this object, as if by assignment; the
     * contents of the fields are not themselves cloned. Thus, this method
     * performs a "shallow copy" of this object, not a "deep copy" operation.
     * 

     * Object类本身没有实现Cloneable接口，所以调用clone方法的如果是Object类的话，会导致在运行时抛出异常。
     * The class {@code Object} does not itself implement the interface
     * {@code Cloneable}, so calling the {@code clone} method on an object
     * whose class is {@code Object} will result in throwing an
     * exception at run time.
     *
     * @return     a clone of this instance.
     * @throws  CloneNotSupportedException  if the object's class does not
     *               support the {@code Cloneable} interface. Subclasses
     *               that override the {@code clone} method can also
     *               throw this exception to indicate that an instance cannot
     *               be cloned.
     * @see java.lang.Cloneable
     */
    protected native Object clone() throws CloneNotSupportedException;

    /**
     * 返回一个表示这个对象的字符串。
     * 通常，该方法返回一个“文本表示”的对象字符串。该结果应该是简洁但是有代表性的易于用户阅读的。
     * Returns a string representation of the object. In general, the
     * {@code toString} method returns a string that
     * "textually represents" this object. The result should
     * be a concise but informative representation that is easy for a
     * person to read.
     * It is recommended that all subclasses override this method.
     * 

     * 该方法返回一个字符串，该字符串包含类名，并且中间用“@”拼接对象的哈希值的未签名的十六进制表示。
     * The {@code toString} method for class {@code Object}
     * returns a string consisting of the name of the class of which the
     * object is an instance, the at-sign character `{@code @}', and
     * the unsigned hexadecimal representation of the hash code of the
     * object. In other words, this method returns a string equal to the
     * value of:
     * 

     * 
     * getClass().getName() + '@' + Integer.toHexString(hashCode())
     * 
     *
     * @return  a string representation of the object.
     */
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

    /**
     * 唤醒一个在这个对象的监视器上等待的线程。
     * 如果在该对象上有很多线程在等待，那么其中一个将被唤醒。选择是任意的，并且在实现的自由裁量权下进行。
     * 一个线程通过调用一个等候方法来等待对象的监视器。
     * Wakes up a single thread that is waiting on this object's
     * monitor. If any threads are waiting on this object, one of them
     * is chosen to be awakened. The choice is arbitrary and occurs at
     * the discretion of the implementation. A thread waits on an object's
     * monitor by calling one of the {@code wait} methods.
     * 

     * 该被唤醒的线程在当前线程释放对当前对象的锁之前是不会继续执行的。
     * 该被唤醒的线程会和其他的任意被激活的线程竞争对当前对象进行加锁；
     * 例如，被唤醒的线程没有任何优势或劣势来决定它是否会成为下一个锁定该对象的线程。即，公平竞争。
     * The awakened thread will not be able to proceed until the current
     * thread relinquishes the lock on this object. The awakened thread will
     * compete in the usual manner with any other threads that might be
     * actively competing to synchronize on this object; for example, the
     * awakened thread enjoys no reliable privilege or disadvantage in being
     * the next thread to lock this object.
     * 

     * 这个方法应该只由一个线程来调用，它是这个对象监视器的所有者。
     * 一个线程成为该对象的监视器的所有者有三种方式：
     * 1、通过执行一个该对象的同步的实例方法。
     * 2、通过执行同步语句的主体，使其与对象同步。
     * 3、如果对象为Class类型，通过执行一个该类的同步静态方法。
     * This method should only be called by a thread that is the owner
     * of this object's monitor. A thread becomes the owner of the
     * object's monitor in one of three ways:
     * 

     * 
By executing a synchronized instance method of that object.
     * 
By executing the body of a {@code synchronized} statement
     *     that synchronizes on the object.
     * 
For objects of type {@code Class,} by executing a
     *     synchronized static method of that class.
     * 
     * 

     * 同一时间，只有一个线程能拥有一个对象的监视器。
     * Only one thread at a time can own an object's monitor.
     *
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.
     * @see        java.lang.Object#notifyAll()
     * @see        java.lang.Object#wait()
     */
    public final native void notify();

    /**
     * 唤醒当前在监视器上等待的所有线程。
     * Wakes up all threads that are waiting on this object's monitor. A
     * thread waits on an object's monitor by calling one of the
     * {@code wait} methods.
     * 

     * The awakened threads will not be able to proceed until the current
     * thread relinquishes the lock on this object. The awakened threads
     * will compete in the usual manner with any other threads that might
     * be actively competing to synchronize on this object; for example,
     * the awakened threads enjoy no reliable privilege or disadvantage in
     * being the next thread to lock this object.
     * 

     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the {@code notify} method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#wait()
     */
    public final native void notifyAll();

    /**
     * 使得当前线程进入等待，直到任何其他线程调用了notify()方法或者notifyAll(),或者设置的等待时间已经结束才会重新唤醒。
     * Causes the current thread to wait until either another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object, or a
     * specified amount of time has elapsed.
     * 

     * 当前线程必须持有这个对象的监视。
     * The current thread must own this object's monitor.
     * 

     * 该方法使得当前线程（暂称为T）将它自己放在等待池中，并且释放所有对该对象的锁。线程T在线程调度中失效并休眠，
     * 直到以下四种情况发生：
     * 1、有其他线程调用了notify()方法，然后线程T被选为要唤醒的线程。
     * 2、有其他线程调用了notifyAll()方法。
     * 3、有其他线程打断了线程T。
     * 3、指定的休眠时间结束后，如果过期时间为0，那么该线程会休眠知道被主动唤醒。
     * This method causes the current thread (call it T) to
     * place itself in the wait set for this object and then to relinquish
     * any and all synchronization claims on this object. Thread T
     * becomes disabled for thread scheduling purposes and lies dormant
     * until one of four things happens:
     * 

     * 
Some other thread invokes the {@code notify} method for this
     * object and thread T happens to be arbitrarily chosen as
     * the thread to be awakened.
     * 
Some other thread invokes the {@code notifyAll} method for this
     * object.
     * 
Some other thread {@linkplain Thread#interrupt() interrupts}
     * thread T.
     * 
The specified amount of real time has elapsed, more or less.  If
     * {@code timeout} is zero, however, then real time is not taken into
     * consideration and the thread simply waits until notified.
     * 
     * 
     * The thread T is then removed from the wait set for this
     * object and re-enabled for thread scheduling. It then competes in the
     * usual manner with other threads for the right to synchronize on the
     * object; once it has gained control of the object, all its
     * synchronization claims on the object are restored to the status quo
     * ante - that is, to the situation as of the time that the {@code wait}
     * method was invoked. Thread T then returns from the
     * invocation of the {@code wait} method. Thus, on return from the
     * {@code wait} method, the synchronization state of the object and of
     * thread {@code T} is exactly as it was when the {@code wait} method
     * was invoked.
     * 

     * A thread can also wake up without being notified, interrupted, or
     * timing out, a so-called spurious wakeup.  While this will rarely
     * occur in practice, applications must guard against it by testing for
     * the condition that should have caused the thread to be awakened, and
     * continuing to wait if the condition is not satisfied.  In other words,
     * waits should always occur in loops, like this one:
     * 
     *     synchronized (obj) {
     *         while (<condition does not hold>)
     *             obj.wait(timeout);
     *         ... // Perform action appropriate to condition
     *     }
     * 
     * (For more information on this topic, see Section 3.2.3 in Doug Lea's
     * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
     * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
     * Language Guide" (Addison-Wesley, 2001).
     *
     * 
If the current thread is {@linkplain java.lang.Thread#interrupt()
     * interrupted} by any thread before or while it is waiting, then an
     * {@code InterruptedException} is thrown.  This exception is not
     * thrown until the lock status of this object has been restored as
     * described above.
     *
     * 

     * Note that the {@code wait} method, as it places the current thread
     * into the wait set for this object, unlocks only this object; any
     * other objects on which the current thread may be synchronized remain
     * locked while the thread waits.
     * 

     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the {@code notify} method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @param      timeout   the maximum time to wait in milliseconds.
     * @throws  IllegalArgumentException      if the value of timeout is
     *               negative.
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @throws  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The interrupted
     *             status of the current thread is cleared when
     *             this exception is thrown.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#notifyAll()
     */
    public final native void wait(long timeout) throws InterruptedException;

    /**
     * 使得当前线程进入等待，直到任何其他线程调用了notify()方法或者notifyAll(),或者设置的等待时间已经结束才会重新唤醒。
     * Causes the current thread to wait until another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object, or
     * some other thread interrupts the current thread, or a certain
     * amount of real time has elapsed.
     * 

     * 该方法和一个入参的wait方法很相似，但是该方法允许对时间的控制更精确，可以精确到纳秒级别。
     * This method is similar to the {@code wait} method of one
     * argument, but it allows finer control over the amount of time to
     * wait for a notification before giving up. The amount of real time,
     * measured in nanoseconds, is given by:
     * 

     * 
     * 1000000*timeout+nanos
     * 

     * 在其他方面，该方法和一个入参的wait方法做的事都一样。特别地，wait(0,0)与wait(0)是一样的。
     * In all other respects, this method does the same thing as the
     * method {@link #wait(long)} of one argument. In particular,
     * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
     * 

     * The current thread must own this object's monitor. The thread
     * releases ownership of this monitor and waits until either of the
     * following two conditions has occurred:
     * 

     * 
Another thread notifies threads waiting on this object's monitor
     *     to wake up either through a call to the {@code notify} method
     *     or the {@code notifyAll} method.
     * 
The timeout period, specified by {@code timeout}
     *     milliseconds plus {@code nanos} nanoseconds arguments, has
     *     elapsed.
     * 
     * 

     * The thread then waits until it can re-obtain ownership of the
     * monitor and resumes execution.
     * 

     * As in the one argument version, interrupts and spurious wakeups are
     * possible, and this method should always be used in a loop:
     * 
     *     synchronized (obj) {
     *         while (<condition does not hold>)
     *             obj.wait(timeout, nanos);
     *         ... // Perform action appropriate to condition
     *     }
     * 
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the {@code notify} method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @param      timeout   the maximum time to wait in milliseconds.
     * @param      nanos      additional time, in nanoseconds range
     *                       0-999999.
     * @throws  IllegalArgumentException      if the value of timeout is
     *                      negative or the value of nanos is
     *                      not in the range 0-999999.
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.
     * @throws  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The interrupted
     *             status of the current thread is cleared when
     *             this exception is thrown.
     */
    public final void wait(long timeout, int nanos) throws InterruptedException {
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos > 0) {
            timeout++;
        }

        wait(timeout);
    }

    /**
     * Causes the current thread to wait until another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object.
     * In other words, this method behaves exactly as if it simply
     * performs the call {@code wait(0)}.
     * 

     * The current thread must own this object's monitor. The thread
     * releases ownership of this monitor and waits until another thread
     * notifies threads waiting on this object's monitor to wake up
     * either through a call to the {@code notify} method or the
     * {@code notifyAll} method. The thread then waits until it can
     * re-obtain ownership of the monitor and resumes execution.
     * 

     * As in the one argument version, interrupts and spurious wakeups are
     * possible, and this method should always be used in a loop:
     * 
     *     synchronized (obj) {
     *         while (<condition does not hold>)
     *             obj.wait();
     *         ... // Perform action appropriate to condition
     *     }
     * 
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the {@code notify} method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @throws  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @throws  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The interrupted
     *             status of the current thread is cleared when
     *             this exception is thrown.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#notifyAll()
     */
    public final void wait() throws InterruptedException {
        wait(0);
    }

    /**
     * 当垃圾收集确定不再有对该对象的引用时，垃圾收集器就会调用垃圾收集器。子类覆盖finalize方法来处理系统资源或执行其他清理工作。
     * 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.
     * 

     * 该方法的通用约定是，当java虚拟机已经断定没有其他对象能够引用它，而且线程还没死亡，除了最后确定的其他对象或类的最后确定所采取的行动之外。
     * 该方法可能采取任何行动，包括使该对象再次可用到其他线程中；该方法普遍的目的是在对象不可逆转之前执行清除操作。
     * The general contract of {@code finalize} is that it is invoked
     * if and when the Java™ 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.
     * 

     * 该方法不执行特殊的操作，它只是正常的返回。Object的子类可能会覆盖这个定义。
     * The {@code finalize} method of class {@code Object} performs no
     * special action; it simply returns normally. Subclasses of
     * {@code Object} may override this definition.
     * 

     * Java编程语言无法保证哪一个线程会调用该方法。但是，当最终被调用时，调用finalize的线程将不会持有任何用户可见的同步锁。
     * 如果该方法抛出一个未捕获的异常，则忽略异常并终止该对象。
     * 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.
     * 

     * 对象的finalize方法被调用后,没有采取进一步的行动,直到Java虚拟机再次确定,不再有任何方法可以访问这个对象的线程还没有死,
     * 包括其他对象或类的可能的行动准备完成,此时对象可能被丢弃。
     * 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.
     * 

     * 该方法被同一个对象的java虚拟机调用的次数不会超过一次。
     * The {@code finalize} method is never invoked more than once by a Java
     * virtual machine for any given object.
     * 

     * 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
     */
    protected void finalize() throws Throwable { }
}