Object源码解析

/**
 * java类继承体系下的根类，是所有类的超类基类，所有的对象都拥有该类的所有方法
 *
 * @author  unascribed
 * @see     java.lang.Class
 * @since   JDK1.0
 */
public class Object {

    /**
     * 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.
     *
     * <p><b>The actual result type is {@code Class<? extends |X|>}
     * where {@code |X|} is the erasure of the static type of the
     * expression on which {@code getClass} is called.</b> For
     * example, no cast is required in this code fragment:</p>
     *
     * <p>
     * {@code Number n = 0;                             }<br>
     * {@code Class<? extends Number> c = n.getClass(); }
     * </p>
     *
     * @return The {@code Class} object that represents the runtime
     *         class of this object.
     * @jls 15.8.2 Class Literals
     * 返回对象运行时类
     */
    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}.
     * <p>
     * The general contract of {@code hashCode} is:
     * <ul>
     * <li>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.
     * <li>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.
     * <li>It is <em>not</em> 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.
     * </ul>
     * <p>
     * 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&trade; programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    public native int hashCode();

    /**
     * Indicates whether some other object is "equal to" this one.
     * 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}).
     * <p>
     * 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
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }

    /**
     * 使用该方法需实现Cloneable接口
     */
    protected native Object clone() throws CloneNotSupportedException;

    /**
     * It is recommended that all subclasses override this method.
     */
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

    /**
     * 唤醒等待此对象监视器的单个线程
     * 同一时间只能有一个线程拥有对象的监视器
     *
     * @throws  IllegalMonitorStateException  如果当前线程不是该对象监视器的所有者
     * @see        java.lang.Object#notifyAll()
     * @see        java.lang.Object#wait()
     */
    public final native void notify();

    /**
     * 唤醒此对象监视器上等待的所有线程。线程通过调用其中一个等待方法来等待对象的监视器。
     * 在当前线程放弃对该对象的锁定之前，唤醒的线程将无法继续。唤醒的线程将以通常的方式与任何其他线程竞争，
     * 这些线程可能正在积极竞争以在此对象上同步；例如，唤醒的线程在成为下一个锁定此对象的线程时没有可靠的特权或劣势。
     *
     * @throws  IllegalMonitorStateException  如果当前线程不是该对象监视器的所有者
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#wait()
     */
    public final native void notifyAll();

    /**
     * 使当前线程等待，直到另一个线程调用此对象的notify（）方法或notifyAll（）方法，或者经过指定的时间。
     * 当前线程必须拥有此对象的监视器。
     *
     * 此方法会导致当前线程（称为t）将自身放置在此对象的等待集中，然后放弃对此对象的任何和所有同步声明。
     * 出于线程调度的目的，线程t将被禁用，并处于休眠状态，直到发生以下四种情况之一：
     *
     * 1.其他一些线程为此对象调用notify方法，而线程t恰好被任意选择为要唤醒的线程。
     * 2.其他一些线程为此对象调用notifyAll方法。
     * 3.另一个线程中断线程t。
     * 4.指定的实时时间或多或少已经过去。但是，如果超时为零，则不考虑实时性，线程只需等待通知。
     *
     * 然后从该对象的等待集中删除线程t，并重新启用线程调度。然后，它以通常的方式与其他线程竞争对象上的同步权；
     * 一旦它获得了对象的控制权，它对该对象的所有同步声明都将恢复到原来的状态，也就是说，恢复到调用wait方法时的状态。
     * 然后线程t从wait方法的调用返回。因此，从wait方法返回时，对象和线程t的同步状态与调用wait方法时完全相同。
     *
     * @see @see 
     * 线程也可以在不被通知、中断或超时的情况下唤醒，即所谓的“虚假唤醒”。虽然这种情况在实践中很少发生，
     * 但应用程序必须通过测试本应导致线程被唤醒的条件，并在条件不满足时继续等待来防范这种情况。
     * 换句话说，等待应该总是以循环的形式出现，就像这样：
     * @see @see
     * 
     * <pre>
     *     synchronized (obj) {
     *         while (&lt;condition does not hold&gt;)
     *             obj.wait(timeout);
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>
     * 
     * (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).
     *
     * <p>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.
     * 
     * 在wait期间，如果当前线程被任何其他线程interrupt，一个InterruptedException会被抛出，
     * 但是被抛出之前，必须先恢复获取锁之前的状态，如果没恢复锁状态，该异常不会抛出
     *
     * 请注意，wait方法在将当前线程放入此对象的等待集中时，仅解锁此对象；
     * 在线程等待期间，可以同步当前线程的任何其他对象将保持锁定状态。
     *
     * 此方法只能由作为此对象监视器所有者的线程调用。有关线程成为监视器所有者的方式的描述，请参见notify方法。
     *
     *
     * @param      timeout   等待的最大毫秒数
     * @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 <i>interrupted
     *             status</i> 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;

    /**
     * @see wait
     *
     * @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 <i>interrupted
     *             status</i> 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);
    }

    /**
     * @see wait
     *
     * @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 <i>interrupted
     *             status</i> 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);
    }

    /**
     * 此方法可被重写
     * 永远不要手动调用，应该由jvm自动决定是否调用
     * 最多只会调用一次
     * 即使该方法报错抛出异常，jvm也不做处理
     */
    protected void finalize() throws Throwable { }
}