jdk源码阅读之-1.1.Object·1
Posted Share猿
tags:
篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了jdk源码阅读之-1.1.Object·1相关的知识，希望对你有一定的参考价值。
1.1.Object小猿“思前享后”为大家分享优质内容！————Share猿
Object是所有类的父类，位于java.lang包中，数组也是它的子类，所以在java中，我们所有的类都可以调用它的方法。
1.1.1.Element
| 版本 | 修饰符 | 类型 | 默认值 | 释义 |
|---|---|---|---|---|
| 无 | 无 | 无 | 无 | 无 |
1.1.2.Constructors
| 版本 | 构造体 | 释义 |
|---|---|---|
| 无 | 无 | 无 |
1.1.3.Method
方法总览
| 版本 | 修饰符 | 方法体 | 参数 | 返回值 | 释义 |
|---|---|---|---|---|---|
| 无 | private static native | registerNatives() | 无 | void | 注册java的方法到c/c++中，形成.dll文件 |
| 无 | public final native | getClass() | 无 | Class<?> | 返回一个对象，在堆内存中生成了一个对象，getclass方法就是获取当前对象所属的字节码文件对象，而不是字节码文件的父类。一个class可以产生好多对象，但是这个对象字节文件只有一个，在内存中只有一份，不可能有第二份。 |
| 无 | public native | hashCode() | 无 | int | hashCode是jdk根据对象的地址或者字符串或者数字算出来的int类型的数值，也就是哈希码，哈希码并不是完全唯一的，它是一种算法，让同一个类的对象按照自己不同的特征尽量的有不同的哈希码，但不表示不同的对象哈希码完全不同。hashCode返回对象的内存地址经过处理后的结构，由于每个对象的内存地址都不一样，所以哈希码也不一样。 |
| 无 | public | equals(Object obj) | Object | boolean | Object默认的是判断该对象的文件字节码是否相等，所以如果我们不重写方法，只要是同一个类，他的不同对象都相等。当然我们可以重写该对象，如果内容相等才相等。 |
| 无 | protected native | clone() | 无 | Object | 在源码中我们可以看到，如果类没有实现Cloneable接口，就会抛出CloneNotSupportedException异常，因为这个方法是被protected修饰过的只能在lang包下的类被调用。几乎所有的数组都是实现了Cloneable接口。这个方法适合一个“浅拷贝”不适合深拷贝。x.clone() != x是true，x.clone().getClass() == x.getClass()为true，x.clone().equals(x)为true |
| 无 | public | toString() | 无 | String | toString方法返回:getClass().getName() + '@' + Integer.toHexString(hashCode()) |
| 无 | final native | notify() | 无 | void | 这个方法的作用是唤醒该对象的进程。 |
| 无 | public final native | notifyAll() | 无 | void | 唤醒对象的所有monitor |
| 无 | public final native | wait(long timeout) | long timeout | void | 对象的一把锁，可以给某个线程开一个监视器监视该对象也可以给所有线程开一个监视器去竞争该对象的状态。 |
| 无 | public final | wait(long timeout, int nanos) | long timeout, int nanos | void | 导致当前线程等待，直到其他线程调用此对象的 notify() 方法或notifyAll()方法，或在指定已经过去的时间。此方法类似于wait方法的一个参数，但它允许更好地控制的时间等待一个通知放弃之前的量。实时量，以毫微秒计算，计算公式如下：1000000*timeout+nanos |
| 无 | public final | wait() | 无 | void | 学习了上面两个方法这个方法就不再讲解了 |
| 无 | protected | finalize() | 无 | void | 垃圾回收器准备释放内存的时候，会先调用finalize() |
1.1.3.1.registerNatives()
private static native void registerNatives(); static {
registerNatives();
}从上述代码我们可以看到该方法默认是private，不过在静态的代码块中被调用，也就是初始化的时候就被调用了。
1.1.3.2.getClass()
/** * 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 */
public final native Class<?> getClass();1.1.3.3. hashCode()
/** * 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™ 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();1.1.3.4. equals(Object obj)
/** * 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 */
public boolean equals(Object obj) { return (this == obj);
}1.1.3.5. clone()
/** * 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 */
protected native Object clone() throws CloneNotSupportedException;1.1.3.6. toString()
/** * 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. */
public String toString() { return getClass().getName() + "@" + Integer.toHexString(hashCode());
}1.1.3.7. notify()
/** * 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() */
public final native void notify();Object是自带锁的，通过这个方法我们可以唤醒该对象的object's monitor，解锁后，线程就可以去竞争这个对象了。
1.1.3.8. notifyAll()
/** * 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() */
public final native void notifyAll();唤醒所有等待此对象监视器的线程。我们可以通过wait方法去开启一个对象的监视器。唤醒的线程将不能继续进行，直到当前线程线程放弃此对象上的锁。唤醒线程将以通常的方式与任何其他线程进行竞争积极竞争同步这个对象
1.1.3.9. wait(long timeout)
/** * 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. * <p> * The current thread must own this object's monitor. * <p> * This method causes the current thread (call it <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. Thread <var>T</var> * becomes disabled for thread scheduling purposes and lies dormant * until one of four things happens: * <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. If * {@code timeout} is zero, however, then real time is not taken into * consideration and the thread simply waits until notified. * </ul> * The thread <var>T</var> 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 <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 also wake up without being notified, interrupted, or * timing out, a so-called <i>spurious wakeup</i>. 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: * <pre> * synchronized (obj) { * while (<condition does not hold>) * 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. * * <p> * 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. * <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. * * @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 <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;导致当前线程等待，直到另一个线程调用，notify和notifyAll都可以唤醒该进程的监控器或者制定的时间过去了激活该进程的监控器。当前线程必须拥有这个对象的监视器。线程也可以通过interrupted或者时间过期被打断。
1.1.3.10.wait(long timeout, int nanos)
/** * 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. * <p> * 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: * <blockquote> * <pre> * 1000000*timeout+nanos</pre></blockquote> * <p> * 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)}. * <p> * 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: * <ul> * <li>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. * <li>The timeout period, specified by {@code timeout} * milliseconds plus {@code nanos} nanoseconds arguments, has * elapsed. * </ul> * <p> * The thread then waits until it can re-obtain ownership of the * monitor and resumes execution. * <p> * As in the one argument version, interrupts and spurious wakeups are * possible, and this method should always be used in a loop: * <pre> * synchronized (obj) { * while (<condition does not hold>) * obj.wait(timeout, nanos); * ... // Perform action appropriate to condition * } * </pre> * 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 <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);
}始终搞不懂这个方法多了一个参数nanos有什么卵用，看了代码可以理解了，我们知道timout的参数单位是毫秒级的，如果加了nanos参数，大于零则加1毫秒，这让时间更加精确。
1.1.3.11.wait()
/** * 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)}. * <p> * 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. * <p> * As in the one argument version, interrupts and spurious wakeups are * possible, and this method should always be used in a loop: * <pre> * synchronized (obj) { * while (<condition does not hold>) * obj.wait(); * ... // Perform action appropriate to condition * } * </pre> * 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 <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);
}看了之前的几个方法，看到这个方法想必我们很清楚了。
1.1.3.12.finalize()
/** * 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™ 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 * @see java.lang.ref.WeakReference * @see java.lang.ref.PhantomReference * @jls 12.6 Finalization of Class Instances */
protected void finalize() throws Throwable { }当垃圾回收器将要释放一个对象的内存时，它调用该对象的finalize() 方法（如果该对象定义了此方法）。垃圾回收器以独立的低优先级的方式运行，只有当其他线程挂起等待该内存释放的情况出现时，它才开始运行释放对象的内存。（事实上，你可以调用System.gc() 方法强制垃圾回收器来释放这些对象的内存。
1.1.4.Question
1.1.4.1.为什么同一个class不同对象通过getClass方法得到的值是一样的？
public class ObjectTest { public static void main(String [] args){
Book book1=new Book(5l, subDate(0), "zjq5");
Book book2=new Book(3l, subDate(2), "zjq3");
System.out.println(book1.getClass().equals(book2.getClass()));
} public static Date subDate(int sum){ return new Date(new Date().getTime()-sum*24*60*60*1000);
}
}因为class的文件的字节码是一样的，而且在内存中仅此一份，所以同一个class不同对象得到的class是一样，实际上getClass()得到的是该类的字节码文件。
1.1.4.2.为什么对象要进行hashCode？？？
这样做可以提高程序查询的效率，为什么这么说那？？假如我们要判断两个对象是否一样，如果不hash我们只能一个一个去比较，但是hash之后得到的就是一串数字，只需要比较这一串数字是否相等即可。
1.1.4.3.clone一个对象和new一个对象有什么区别？？？
文档中有写的不合适的地方，烦请联系我的微信（nongjiachuangzhan）或QQ（330334064）帮忙指正共同完善文档，谢谢！
相关链接：
【1】Java(TM) EE 8 Specification APIs·Oracle
【2】Java中Object的finalize()方法·Thuantanon
【3】Java finalize方法使用
文档会不定期的更新和修改，最新文档请点击↓↓↓↓阅读原文↓↓↓↓
以上是关于jdk源码阅读之-1.1.Object·1的主要内容，如果未能解决你的问题，请参考以下文章