【Java】对象的拷贝

几个概念

浅拷贝

基本数据类型拷贝数值，而对象拷贝对象的引用。

深拷贝

基本数据类型拷贝数值，而对象，则创建一个新的对象，并把原本对象里面的值一一拷贝到新的对象中，开辟了新的内存空间，而不仅仅引用同一个对象。

native

JNI是Java Native Interface的 缩写。从Java 1.1开始，Java Native Interface (JNI)标准成为java平台的一部分，它允许Java代码和其他语言写的代码进行交互。JNI一开始是为了本地已编译语言，尤其是C和C++而设计的，但是它并不妨碍你使用其他语言，只要调用约定受支持就可以了。使用java与本地已编译的代码交互，通常会丧失平台可移植性。但是，有些情况下这样做是可以接受的，甚至是必须的，比如，使用一些旧的库，与硬件、操作系统进行交互，或者为了提高程序的性能。JNI标准至少保证本地代码能工作在任何Java 虚拟机实现下。

Object的clone方法

自定义类实现clone方法的要求

• 继承Object
  因为Java中除了极个别的类之外，都是继承自Object，所以都从Object中继承了clone方法。
• 实现Cloneable接口
  Cloneable并没有声明的方法，所以这个接口相当于一个“标记”的作用，标记该类可以调用clone方法。如果某些类不需要实现clone，就可以不实现该接口，从而达到一个保护的作用。如果没有实现该接口，而去调用clone方法的话则会抛出CloneNotSupportedException。
• 将clone方法声明为public
• 如果有必要重写clone方法，则需要调用super.clone()

几个要点

• Object中clone方法只通过JNI实现对象浅拷贝。
  
  • x.clone() != x
    返回的拷贝对象是开辟新的内存。
  • x.clone().getClass() == x.getClass()
  • x.clone().equals(x)
    “值相等”只是基本数据类型的值相等，而引用类型的引用相等。如果该类的成员对象只是基本数据类型以及不可变对象的引用类型，就无须重写clone方法，只需要把修饰符改成public。但是所以如果要实现深拷贝，则需要重写clone方法。而引用类型的深拷贝的实现同样按照以上的要求——实现Cloneable接口。
• 如果没有实现Cloneable接口，而去调用clone方法的话则会抛出CloneNotSupportedException。
• 数组默认实现Cloneable接口，实现浅拷贝。

Object的clone方法

public class Object {/**     * 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;}

接口Cloneable

/** * A class implements the <code>Cloneable</code> interface to * indicate to the {@link java.lang.Object#clone()} method that it * is legal for that method to make a * field-for-field copy of instances of that class. * <p> * Invoking Object's clone method on an instance that does not implement the * <code>Cloneable</code> interface results in the exception * <code>CloneNotSupportedException</code> being thrown. * <p> * By convention, classes that implement this interface should override * <tt>Object.clone</tt> (which is protected) with a public method. * See {@link java.lang.Object#clone()} for details on overriding this * method. * <p> * Note that this interface does <i>not</i> contain the <tt>clone</tt> method. * Therefore, it is not possible to clone an object merely by virtue of the * fact that it implements this interface.  Even if the clone method is invoked * reflectively, there is no guarantee that it will succeed. * * @author  unascribed * @see     java.lang.CloneNotSupportedException * @see     java.lang.Object#clone() * @since   JDK1.0 */public interface Cloneable {}

利用串行化来实现深层复制

在Java语言里深层复制一个对象，常常可以先使对象实现Serializable接口，然后把对象（实际上只是对象的一个拷贝）写到一个流中，再从流中读出来，便可以重建对象。