免费视频淫片aa毛片_日韩高清在线亚洲专区vr_日韩大片免费观看视频播放_亚洲欧美国产精品完整版

Part I

沒啥好說的，直接開始Part II吧。

Part II

談到了對象的克隆，就不得不說為什么要對對象進行克隆。Java中所有的對象都是保存在堆中，而堆是供全局共享的。也就是說，如果同一個Java程序的不同方法，只要能拿到某個對象的引用，引用者就可以隨意的修改對象的內部數據（前提是這個對象的內部數據通過get/set方法曝露出來）。有的時候，我們編寫的代碼想讓調用者只獲得該對象的一個拷貝（也就是一個內容完全相同的對象，但是在內存中存在兩個這樣的對象），有什么辦法可以做到呢？當然是克隆咯。

Part III

首先，我們是程序員，當然是用我們程序員的語言來交流。

import java.util.Date;
public class User implements Cloneable {
	private String username;
	private String password;
	private Date birthdate;
	public User(String username, String password, Date birthdate) {
		this.username = username;
		this.password = password;
		this.birthdate = birthdate;
	}
	@Override
	protected Object clone() throws CloneNotSupportedException {
		return super.clone();
	}
	@Override
	public int hashCode() {
		// 省略equals的實現(可用eclipse自動生成)
	}
	@Override
	public boolean equals(Object obj) {
		// 省略equals的實現(可用eclipse自動生成)
	}
	// 省略一大堆get/set方法
}

上述代碼構建了一個User類，并且實現了java.lang.Cloneable接口。顧名思義，Cloneable的意思就是說明這個類可以被克隆的意思。

而我們先去看看java.lang.Cloneable這個接口有些什么。

/*
 * @(#)Cloneable.java	1.17 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package java.lang;
/**
 * A class implements the Cloneable 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. 
 * 
 
 * Invoking Object's clone method on an instance that does not implement the 
 * Cloneable interface results in the exception 
 * CloneNotSupportedException being thrown.
 * 
 
 * By convention, classes that implement this interface should override 
 * Object.clone (which is protected) with a public method.
 * See {@link java.lang.Object#clone()} for details on overriding this
 * method.
 * 
 
 * Note that this interface does not contain the clone 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
 * @version 1.17, 11/17/05
 * @see     java.lang.CloneNotSupportedException
 * @see     java.lang.Object#clone()
 * @since   JDK1.0
 */
public interface Cloneable { 
}

不要驚訝，沒錯，除了一大堆的雞腸以外，這個接口沒有定義任何的方法簽名。也就是說，我們要克隆一個對象，但是他又不給我提供一個方法。那該怎么辦呢？不怕，我們還有全能的Object類，別忘記他可是所有類的始祖?。ㄉ褚话愕拇嬖谥?，所以，有事沒事都該去問候一下他老人家。

/*
 * @(#)Object.java	1.73 06/03/30
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package java.lang;
/**
 * Class Object is the root of the class hierarchy. 
 * Every class has Object as a superclass. All objects, 
 * including arrays, implement the methods of this class. 
 *
 * @author  unascribed
 * @version 1.73, 03/30/06
 * @see     java.lang.Class
 * @since   JDK1.0
 */
public class Object {
    
   // 省略N多的代碼
    /**
     * 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 x, the expression:
     * 
     * 
     * x.clone() != x
     * will be true, and that the expression:
     * 
     * 
     * x.clone().getClass() == x.getClass()
     * will be true, but these are not absolute requirements. 
     * While it is typically the case that:
     * 
     * 
     * x.clone().equals(x)
     * will be true, this is not an absolute requirement. 
     * 
 
     * By convention, the returned object should be obtained by calling
     * super.clone.  If a class and all of its superclasses (except
     * Object) obey this convention, it will be the case that
     * x.clone().getClass() == x.getClass().
     * 
 
     * 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 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 super.clone
     * need to be modified.
     * 
 
     * The method clone for class Object performs a 
     * specific cloning operation. First, if the class of this object does 
     * not implement the interface Cloneable, then a 
     * CloneNotSupportedException is thrown. Note that all arrays 
     * are considered to implement the interface Cloneable. 
     * 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.
     * 
 
     * The class Object does not itself implement the interface 
     * Cloneable, so calling the clone method on an object 
     * whose class is Object will result in throwing an
     * exception at run time.
     *
     * @return     a clone of this instance.
     * @exception  CloneNotSupportedException  if the object's class does not
     *               support the Cloneable interface. Subclasses
     *               that override the 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;
}

呵呵，又是一大串的雞腸，別以為我是來湊字數的，這些都是Sun公司Java開發(fā)人員寫的技術文章，多看看少說話吧。

沒錯，又是個native方法，果然是個高深的東西，不過我們還是要占一下他的便宜。而且他這個方法是protected的，分明就是叫我們去占便宜的。

再繼續(xù)看看下面測試代碼。

import java.util.Date;
import org.junit.Test;
public class TestCase {
	
	@Test
	public void testUserClone() throws CloneNotSupportedException {
		User u1 = new User("Kent", "123456", new Date());
		User u2 = u1;
		User u3 = (User) u1.clone();
		
		System.out.println(u1 == u2);		// true
		System.out.println(u1.equals(u2));	// true
		
		System.out.println(u1 == u3);		// false
		System.out.println(u1.equals(u3));	// true
	}
	
}

這個clone()方法果然牛，一下子就把我們的對象克隆了一份，執(zhí)行結果也符合我們的預期，u1和u3的地址不同但是內容相同。

Part IV

通過上述的例子，我們可以看出，要讓一個對象進行克隆，其實就是兩個步驟：

1. 讓該類實現java.lang.Cloneable接口；

2. 重寫（override）Object類的clone()方法。

但是，事實上真的是如此簡單嗎？再看下面的代碼。

public class Administrator implements Cloneable {
	private User user;
	private Boolean editable;
	public Administrator(User user, Boolean editable) {
		this.user = user;
		this.editable = editable;
	}
	
	@Override
	protected Object clone() throws CloneNotSupportedException {
		return super.clone();
	}
	@Override
	public int hashCode() {
		// 老規(guī)矩
	}
	@Override
	public boolean equals(Object obj) {
		// 老規(guī)矩
	}
	// 老規(guī)矩
}

上面定義了一個Administrator類，這個類持有一個User類的對象。接下來我們看看對Administrator對象進行克隆會有什么效果。

import java.util.Date;
import org.junit.Test;
public class TestCase {
	@Test
	public void testAdministratorClone() throws CloneNotSupportedException {
		Administrator a1 = new Administrator(new User("Kent", "123456", new Date()), true);
		Administrator a2 = a1;
		Administrator a3 = (Administrator) a1.clone();
		
		System.out.println(a1 == a2);			// true
		System.out.println(a1.equals(a2));		// true
		
		System.out.println(a1 == a3);			// false
		System.out.println(a1.equals(a3));		// true
		
		System.out.println(a1.getUser() == a3.getUser());		//true ! It's not our expected!!!!!
		System.out.println(a1.getUser().equals(a3.getUser()));	//true
	}
}

呵呵呵！出問題了吧。Java哪是那么容易就能駕馭的說！

這里我們就可以引入兩個專業(yè)的術語：淺克?。╯hallow clone）和深克?。╠eep clone）。

所謂的淺克隆，顧名思義就是很表面的很表層的克隆，如果我們要克隆Administrator對象，只克隆他自身以及他包含的所有對象的引用地址。

而深克隆，就是非淺克隆。克隆除自身以外所有的對象，包括自身所包含的所有對象實例。至于深克隆的層次，由具體的需求決定，也有“N層克隆”一說。

但是，所有的基本（primitive）類型數據，無論是淺克隆還是深克隆，都會進行原值克隆。畢竟他們都不是對象，不是存儲在堆中。注意：基本數據類型并不包括他們對應的包裝類。

如果我們想讓對象進行深度克隆，我們可以這樣修改Administrator類。

@Override
protected Object clone() throws CloneNotSupportedException {
	Administrator admin = (Administrator) super.clone();
	admin.user = (User) admin.user.clone();
	return admin;
}

由于Boolean會對值進行緩存處理，所以我們沒必要對Boolean的對象進行克隆。并且Boolean類也沒有實現java.lang.Cloneable接口。

Part V

1. 讓該類實現java.lang.Cloneable接口；

2. 確認持有的對象是否實現java.lang.Cloneable接口并提供clone()方法；

3. 重寫（override）Object類的clone()方法，并且在方法內部調用持有對象的clone()方法；

4. ……

5. 多麻煩啊，調來調去的，如果有N多個持有的對象，那就要寫N多的方法，突然改變了類的結構，還要重新修改clone()方法。

難道就沒有更好的辦法嗎？

Part VI

接下來要重點介紹一下使用java.lang.Serializable來實現對象的深度克隆。

首先，我們編寫一個工具類并提供cloneTo()方法。

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
public abstract class BeanUtil {
	@SuppressWarnings("unchecked")
	public static  T cloneTo(T src) throws RuntimeException {
		ByteArrayOutputStream memoryBuffer = new ByteArrayOutputStream();
		ObjectOutputStream out = null;
		ObjectInputStream in = null;
		T dist = null;
		try {
			out = new ObjectOutputStream(memoryBuffer);
			out.writeObject(src);
			out.flush();
			in = new ObjectInputStream(new ByteArrayInputStream(memoryBuffer.toByteArray()));
			dist = (T) in.readObject();
		} catch (Exception e) {
			throw new RuntimeException(e);
		} finally {
			if (out != null)
				try {
					out.close();
					out = null;
				} catch (IOException e) {
					throw new RuntimeException(e);
				}
			if (in != null)
				try {
					in.close();
					in = null;
				} catch (IOException e) {
					throw new RuntimeException(e);
				}
		}
		return dist;
	}
}

看不懂，沒關系，直接拿去用就可以了。嘻嘻。

接下來我們測試一下是否能通過這個工具來實現深度克隆。

又是這個可愛的TestCase，可憐的每次都要動他……

import java.util.Date;
import org.junit.Test;
public class TestCase {
	
	@Test
	public void testCloneTo() {
		Administrator src = new Administrator(new User("Kent", "123456", new Date()), true);
		Administrator dist = BeanUtil.cloneTo(src);
		
		System.out.println(src == dist);			// false
		System.out.println(src.equals(dist));		// true
		
		System.out.println(src.getUser() == dist.getUser());		//false ! Well done!
		System.out.println(src.getUser().equals(dist.getUser()));	//true
	}
	
}

好了，無論你的對象有多么的復雜，只要這些對象都能夠實現java.lang.Serializable接口，就可以進行克隆，而且這種克隆的機制是JVM完成的，不需要修改實體類的代碼，方便多了。

為什么這么簡單就可以實現對象的克隆呢？java.lang.Serializable接口又是干嘛用的呢？如果想知道這些問題的答案，請匯款到0123 4567 8901 2345678。

……

開玩笑咔……這些問題的答案，我都留在了java.lang.Serializable的專題里面，請各位翻一下我的日志吧。PS.有可能這幾天之內會出來，看哪天無聊寫寫吧……先睡夠再說……