Object.java

/*
 * Copyright (c) 1994, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.lang;

import jdk.internal.HotSpotIntrinsicCandidate;

/**
 * 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 1.0
 */
// Java类的共同祖先
public class Object {
    
    static {
        registerNatives();
    }
    
    
    /**
     * Constructs a new object.
     */
    @HotSpotIntrinsicCandidate
    public 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
     */
    // 返回当前对象所属的类的类对象
    @HotSpotIntrinsicCandidate
    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. (The hashCode may or may not be implemented
     * as some function of an object's memory address at some point
     * in time.)
     *
     * @return a hash code value for this object.
     *
     * @see java.lang.Object#equals(java.lang.Object)
     * @see java.lang.System#identityHashCode
     */
    // 返回当前对象的哈希码，往往作为当前对象的唯一标识，参见System.identityHashCode(Object)和Arrays.hashCode()方法
    @HotSpotIntrinsicCandidate
    public native int hashCode();
    
    /**
     * Indicates whether some other object is "equal to" this one.
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     * {@code x}, {@code x.equals(x)} should return
     * {@code true}.
     * <li>It is <i>symmetric</i>: 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}.
     * <li>It is <i>transitive</i>: 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}.
     * <li>It is <i>consistent</i>: 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.
     * <li>For any non-null reference value {@code x},
     * {@code x.equals(null)} should return {@code false}.
     * </ul>
     * <p>
     * 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
     */
    // 判等，默认的实现只是简单地比较两个对象的引用。更多判等操作可参考Arrays.equals方法
    public boolean equals(Object obj) {
        return (this == obj);
    }
    
    /**
     * 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.
     * <p>
     * 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:
     * <blockquote>
     * <pre>
     * getClass().getName() + '@' + Integer.toHexString(hashCode())
     * </pre></blockquote>
     *
     * @return a string representation of the object.
     */
    // 字符串化，往往需要重写
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }
    
    /**
     * 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:
     * <blockquote>
     * <pre>
     * x.clone() != x</pre></blockquote>
     * will be true, and that the expression:
     * <blockquote>
     * <pre>
     * x.clone().getClass() == x.getClass()</pre></blockquote>
     * will be {@code true}, but these are not absolute requirements.
     * While it is typically the case that:
     * <blockquote>
     * <pre>
     * x.clone().equals(x)</pre></blockquote>
     * will be {@code true}, this is not an absolute requirement.
     * <p>
     * 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()}.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * 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
     */
    /*
     * 浅拷贝，使用时往往需要重写为public形式。
     * 注：要求被克隆的对象所属的类实现Cloneable接口
     */
    @HotSpotIntrinsicCandidate
    protected native Object clone() throws CloneNotSupportedException;
    
    /**
     * 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.
     * <p>
     * The general contract of {@code finalize} is that it is invoked
     * if and when the Java&trade; 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.
     * <p>
     * The {@code finalize} method of class {@code Object} performs no
     * special action; it simply returns normally. Subclasses of
     * {@code Object} may override this definition.
     * <p>
     * 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.
     * <p>
     * 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.
     * <p>
     * The {@code finalize} method is never invoked more than once by a Java
     * virtual machine for any given object.
     * <p>
     * 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
     * @apiNote Classes that embed non-heap resources have many options
     * for cleanup of those resources. The class must ensure that the
     * lifetime of each instance is longer than that of any resource it embeds.
     * {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
     * objects remain reachable while resources embedded in the object are in use.
     * <p>
     * A subclass should avoid overriding the {@code finalize} method
     * unless the subclass embeds non-heap resources that must be cleaned up
     * before the instance is collected.
     * Finalizer invocations are not automatically chained, unlike constructors.
     * If a subclass overrides {@code finalize} it must invoke the superclass
     * finalizer explicitly.
     * To guard against exceptions prematurely terminating the finalize chain,
     * the subclass should use a {@code try-finally} block to ensure
     * {@code super.finalize()} is always invoked. For example,
     * <pre>{@code      @Override
     *     protected void finalize() throws Throwable {
     *         try {
     *             ... // cleanup subclass state
     *         } finally {
     *             super.finalize();
     *         }
     *     }
     * }</pre>
     * @jls 12.6 Finalization of Class Instances
     * @see java.lang.ref.WeakReference
     * @see java.lang.ref.PhantomReference
     * @deprecated The finalization mechanism is inherently problematic.
     * Finalization can lead to performance issues, deadlocks, and hangs.
     * Errors in finalizers can lead to resource leaks; there is no way to cancel
     * finalization if it is no longer necessary; and no ordering is specified
     * among calls to {@code finalize} methods of different objects.
     * Furthermore, there are no guarantees regarding the timing of finalization.
     * The {@code finalize} method might be called on a finalizable object
     * only after an indefinite delay, if at all.
     *
     * Classes whose instances hold non-heap resources should provide a method
     * to enable explicit release of those resources, and they should also
     * implement {@link AutoCloseable} if appropriate.
     * The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}
     * provide more flexible and efficient ways to release resources when an object
     * becomes unreachable.
     */
    // 对象在被GC回收后执行的清理操作。可能会引发OOM，建议使用java.lang.ref.Cleaner替代
    @Deprecated(since = "9")
    protected void finalize() throws Throwable {
    }
    
    
    
    /*▼ 线程 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /*
     * wait使得调用wait方法的线程放弃锁的持有权，并进入WAITING或TIMED_WAITING状态
     *
     * wait方法应当配合synchronized一起使用：
     *
     * 示例一：
     * synchronized void fun(){
     *   try {
     *      wait(1000);
     *   } catch(InterruptedException e) {
     *      e.printStackTrace();
     *   }
     * }
     *
     * 示例二：
     * synchronized(object) {
     *   try {
     *     object.wait(1000);
     *   } catch(InterruptedException e) {
     *     e.printStackTrace();
     *   }
     * }
     *
     * wait让当前线程陷入等待的同时，释放其持有的锁，以便让其他线程争夺锁的控制权
     * 作为对比，Thread.sleep方法即使陷入等待，也不会释放其持有的锁
     *
     * wait线程醒来的条件：
     * 1. 超时
     * 2. 被notify()或notifyAll()唤醒
     * 3. 在其他线程中调用该线程的interrupt()方法
     *
     * 注：
     * wait方法持有的锁是当前wait所处的上下文的对象（某个栈帧中的对象）
     * 如果wait持有的锁与当前上下文中的锁不一致，或者wait和notify用的锁不一致，会触发InterruptedException
     */
    
    /**
     * Causes the current thread to wait until it is awakened, typically
     * by being <em>notified</em> or <em>interrupted</em>.
     * <p>
     * In all respects, this method behaves as if {@code wait(0L, 0)}
     * had been called. See the specification of the {@link #wait(long, int)} method
     * for details.
     *
     * @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. The <em>interrupted status</em> of the
     *                                      current thread is cleared when this exception is thrown.
     * @see #notify()
     * @see #notifyAll()
     * @see #wait(long)
     * @see #wait(long, int)
     */
    // 永不超时，需要靠上述条件2或条件3唤醒（释放锁）
    public final void wait() throws InterruptedException {
        wait(0L);
    }
    
    /**
     * Causes the current thread to wait until it is awakened, typically
     * by being <em>notified</em> or <em>interrupted</em>, or until a
     * certain amount of real time has elapsed.
     * <p>
     * In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
     * had been called. See the specification of the {@link #wait(long, int)} method
     * for details.
     *
     * @param timeoutMillis the maximum time to wait, in milliseconds
     *
     * @throws IllegalArgumentException     if {@code timeoutMillis} 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. The <em>interrupted status</em> of the
     *                                      current thread is cleared when this exception is thrown.
     * @see #notify()
     * @see #notifyAll()
     * @see #wait()
     * @see #wait(long, int)
     */
    // 等待timeoutMillis毫秒之后自动醒来，或者靠上述条件2或条件3唤醒（释放锁）
    public final native void wait(long timeoutMillis) throws InterruptedException;
    
    /**
     * Causes the current thread to wait until it is awakened, typically
     * by being <em>notified</em> or <em>interrupted</em>, or until a
     * certain amount of real time has elapsed.
     * <p>
     * The current thread must own this object's monitor lock. See the
     * {@link #notify notify} method for a description of the ways in which
     * a thread can become the owner of a monitor lock.
     * <p>
     * This method causes the current thread (referred to here as <var>T</var>) to
     * place itself in the wait set for this object and then to relinquish any
     * and all synchronization claims on this object. Note that only the locks
     * on this object are relinquished; any other objects on which the current
     * thread may be synchronized remain locked while the thread waits.
     * <p>
     * Thread <var>T</var> then becomes disabled for thread scheduling purposes
     * and lies dormant until one of the following occurs:
     * <ul>
     * <li>Some other thread invokes the {@code notify} method for this
     * object and thread <var>T</var> happens to be arbitrarily chosen as
     * the thread to be awakened.
     * <li>Some other thread invokes the {@code notifyAll} method for this
     * object.
     * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
     * thread <var>T</var>.
     * <li>The specified amount of real time has elapsed, more or less.
     * The amount of real time, in nanoseconds, is given by the expression
     * {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
     * are both zero, then real time is not taken into consideration and the
     * thread waits until awakened by one of the other causes.
     * <li>Thread <var>T</var> is awakened spuriously. (See below.)
     * </ul>
     * <p>
     * The thread <var>T</var> is then removed from the wait set for this
     * object and re-enabled for thread scheduling. It competes in the
     * usual manner with other threads for the right to synchronize on the
     * object; once it has regained 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 <var>T</var> 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.
     * <p>
     * A thread can wake up without being notified, interrupted, or timing out, a
     * so-called <em>spurious wakeup</em>.  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. See the example below.
     * <p>
     * For more information on this topic, see section 14.2,
     * "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
     * in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
     * Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
     * 2008).
     * <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.  The <em>interrupted status</em> of the current thread is cleared when
     * this exception is thrown. This exception is not thrown until the lock status of
     * this object has been restored as described above.
     *
     * @param timeoutMillis the maximum time to wait, in milliseconds
     * @param nanos         additional time, in nanoseconds, in the range range 0-999999 inclusive
     *
     * @throws IllegalArgumentException     if {@code timeoutMillis} is negative,
     *                                      or if the value of {@code nanos} is out of range
     * @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. The <em>interrupted status</em> of the
     *                                      current thread is cleared when this exception is thrown.
     * @apiNote The recommended approach to waiting is to check the condition being awaited in
     * a {@code while} loop around the call to {@code wait}, as shown in the example
     * below. Among other things, this approach avoids problems that can be caused
     * by spurious wakeups.
     *
     * <pre>{@code
     *     synchronized (obj) {
     *         while (<condition does not hold> and <timeout not exceeded>) {
     *             long timeoutMillis = ... ; // recompute timeout values
     *             int nanos = ... ;
     *             obj.wait(timeoutMillis, nanos);
     *         }
     *         ... // Perform action appropriate to condition or timeout
     *     }
     * }</pre>
     * @see #notify()
     * @see #notifyAll()
     * @see #wait()
     * @see #wait(long)
     */
    /*
     * 至少等待timeoutMillis毫秒，nanos是一个纳秒级的附加时间，用来微调timeoutMillis参数（释放锁）
     * 内部实现可参考Thread中的void sleep(long millis, int nanos)方法
     */
    public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
        if(timeoutMillis<0) {
            throw new IllegalArgumentException("timeoutMillis value is negative");
        }
        
        if(nanos<0 || nanos>999999) {
            throw new IllegalArgumentException("nanosecond timeout value out of range");
        }
        
        if(nanos>0) {
            timeoutMillis++;
        }
        
        wait(timeoutMillis);
    }
    
    /**
     * 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.
     * <p>
     * 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.
     * <p>
     * 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:
     * <ul>
     * <li>By executing a synchronized instance method of that object.
     * <li>By executing the body of a {@code synchronized} statement
     * that synchronizes on the object.
     * <li>For objects of type {@code Class,} by executing a
     * synchronized static method of that class.
     * </ul>
     * <p>
     * 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()
     */
    // 随机唤醒某个具有相同锁的对象从wait状态进入争锁状态
    @HotSpotIntrinsicCandidate
    public final native void notify();
    
    /**
     * 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.
     * <p>
     * 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.
     * <p>
     * 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()
     */
    // 唤醒所有具有相同锁的对象从wait状态进入争锁状态
    @HotSpotIntrinsicCandidate
    public final native void notifyAll();
    
    /*▲ 线程 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    private static native void registerNatives();
    
}