前言
在 Java 語言中，我們說兩個對象是否相等通常有兩層含義：
對象的內容是否相等，通常使用到對象的
equals(Object o)函數；引用的地址是否相同，使用運算符
＝＝比較即可。
當兩個對象通過賦值符號 = 賦值時，表明這兩個對象指向了內存中同一個地址，所以改變其中一個對象的內容，也就間接地改變了另一個對象的內容。有時候，我們需要從一個已經存在的對象重新拷貝一份出來，并且不僅這兩個對象內容相等，在內存中存在兩個獨立的存儲地址，互不影響，這時，就需要用到 Java 中的克隆機制。
Cloneable
通過 Cloneable 接口可以很輕松地實現 Java 對象的克隆，只需要 implements Cloneable 并實現 Object 的 clone() 方法即可，如：
public class User implements Cloneable{
private String username;
private String password;
public User(String username, String password) {
super();
this.username = username;
this.password = password;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
@Override
public boolean equals(Object obj) {
User user = (User) obj;
if (username.equals(user.username) && password.equals(user.password)) {
return true;
}
return false;
}
}
注意這里對象實現的是 Object 類的 clone() 方法，因為 Cloneable 是一個空接口：
package java.lang;
/**
* 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 {
}
從源碼注釋中可以看出，需要實現 Object 類中的 clone() 方法（注意：clone() 函數是一個 native 方法，同時拋出了一個異常）：
/**
* 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;
從 clone() 函數的注釋中能夠看出對象與克隆對象之間的關系，測試代碼如下（注意：我們在 User 對象中重寫了 equals() 函數）：
public static void main(String[] args) throws CloneNotSupportedException{
User userOne, userTwo, userThree;
userOne = new User("username", "password");
userTwo = userOne;
userThree = (User) userOne.clone();
System.out.println(userTwo==userOne); //true
System.out.println(userTwo.equals(userOne)); //true
System.out.println(userThree==userOne); //false
System.out.println(userThree.equals(userOne)); //true
}
測試結果顯示，通過 clone() 函數，我們成功地從 userOne 對象中克隆出了一份獨立的 userThree 對象。
淺克隆與深克隆
談此之前，我們先看一個例子，定義一個名為 Company 的類，并添加一個類型為 User 的成員變量：
public class Company implements Cloneable{
private User user;
private String address;
public Company(User user, String address) {
super();
this.user = user;
this.address = address;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
@Override
public boolean equals(Object obj) {
Company company = (Company) obj;
if (user.equals(company.getUser()) && address.equals(company.address)) {
return true;
}
return false;
}
}
測試代碼及測試結果如下：
public static void main(String[] args) throws CloneNotSupportedException{
Company companyOne, companyTwo, companyThree;
companyOne = new Company(new User("username", "password"), "上海市");
companyTwo = companyOne;
companyThree = (Company) companyOne.clone();
System.out.println(companyTwo==companyOne); //true
System.out.println(companyTwo.equals(companyOne)); //true
System.out.println(companyThree==companyOne); //false
System.out.println(companyThree.equals(companyOne)); //true
System.out.println(companyThree.getUser()==companyOne.getUser()); //true ? 這里為什么不是false呢
System.out.println(companyThree.getUser().equals(companyOne.getUser())); //true
}
問題來了，companyThree 與 companyOne 中的 User 是同一個對象！也就是說 companyThree 只是克隆了 companyOne 的基本數據類型的數據，而對于引用類型的數據沒有進行深度的克隆。也就是俗稱的淺克隆。
淺克隆：顧名思義，就是很表層的克隆，只克隆對象自身的引用地址；
深克隆：也稱“N層克隆”，克隆對象自身以及對象所包含的引用類型對象的引用地址。
這里需要注意的是，對于基本數據類型（primitive）和使用常量池方式創建的String 類型，都會針對原值克隆，所以不存在引用地址一說。當然不包括他們對應的包裝類。
所以使用深克隆就可以解決上述 Company 對象克隆過后兩個 user 對象的引用地址相同的問題。我們修改一下 Company 類的 clone() 函數：
@Override
protected Object clone() throws CloneNotSupportedException {
Company company = (Company) super.clone();
company.user = (User) company.getUser().clone();
return company;
}
再運行測試代碼，就能得到 companyThree.getUser()==companyOne.getUser() 為 false 的結果了。
Serializable實現
通過上述介紹，我們知道，實現一個對象的克隆，需要如下幾步：
對象所在的類實現 Cloneable 接口；
重寫 clone() 函數，如果包涵引用類型的成員變量，需要使用深克隆。
如果對象不包含引用類型成員或者數量少的話，使用 Cloneable 接口還能接受，但當對象包含多個引用類型的成員，同時這些成員又包含了引用類型的成員，那層層克隆豈不是相當繁瑣，并且維護不便？所以，這里介紹一種更加方便的實現方式，使用 ObjectOutputStream 和 ObjectOutputStream 來實現對象的序列化和反序列化：
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
public abstract class BeanUtils {
@SuppressWarnings("unchecked")
public static <T> 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;
}
}
只要要克隆的對象以及對象所包含的引用類型的成員對象所在的類實現了 java.io.Serializable 接口即可做到完美克隆。
