（转）java中的clone技术

编程过程中常常遇到如下情况： 假设有一个对象obj1，在某处需要和obj1一样的实例obj2，强调obj1和obj2是两个独立的实例，只是在开始的时候，它们具有一样的属性。这种情况下，一般的一种解决方法是：重新new一个对象obj2，然后将obj1的属性字段值依次赋予obj2。该种方法可行，但是也比较土。java提供了clone方法，使用clone方法，我们可以高效地解决上述的问题。

       在理解clone方法前，有必要先了解下浅拷贝（shallow copy）和深拷贝（deep copy）。先看一个与浅拷贝相关的代码段：

 

public class FamilyInfo {
    public String address;
    public int memberNum;

    public FamilyInfo(String address, int memberNum) {
        super();
        this.address = address;
        this.memberNum = memberNum;
    }
}

public class FamilyInfo {
	public String address;
	public int memberNum;
	
	public FamilyInfo(String address, int memberNum) {
		super();
		this.address = address;
		this.memberNum = memberNum;
	}
}

 

public class Employee {
    public String name;
    public FamilyInfo familyInfo;

    public Employee(String name, FamilyInfo familyInfo) {
        super();
        this.name = name;
        this.familyInfo = familyInfo;
    }
}

public class Employee {
	public String name;
	public FamilyInfo familyInfo;
	
	public Employee(String name, FamilyInfo familyInfo) {
		super();
		this.name = name;
		this.familyInfo = familyInfo;
	}
}

 

public class CloneTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        FamilyInfo familyInfoA=new FamilyInfo("No.1100 Lianhang Rd.",2);
        Employee employeeA=new Employee("Lily",familyInfoA);
        System.out.println("employeeA's address "+employeeA.familyInfo.address);
        Employee employeeB=employeeA;
        System.out.println("employeeB's address "+employeeB.familyInfo.address);
        System.out.println("---------------------");
        employeeA.familyInfo.address="No.1588 Pulian Rd.";
        System.out.println("employeeA's address "+employeeA.familyInfo.address);
        System.out.println("employeeB's address "+employeeB.familyInfo.address);

    }
}

public class CloneTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		FamilyInfo familyInfoA=new FamilyInfo("No.1100 Lianhang Rd.",2);
		Employee employeeA=new Employee("Lily",familyInfoA);
		System.out.println("employeeA's address "+employeeA.familyInfo.address);
		Employee employeeB=employeeA;
		System.out.println("employeeB's address "+employeeB.familyInfo.address);
		System.out.println("---------------------");
		employeeA.familyInfo.address="No.1588 Pulian Rd.";
		System.out.println("employeeA's address "+employeeA.familyInfo.address);
		System.out.println("employeeB's address "+employeeB.familyInfo.address);	

	}
}

 

 

输出结果为：

 

        我们可以看到，随着employeeA对象 familyInfo值的改变，employeeB对象的值也受到影响。产生这种现象的原因是java中

Employee employeeB=employeeA;这条语句实际上是直接将对象employeeA的引用赋予employeeB，这样两个引用指向的是同一个对象。因此，当emploA对象改变的时候，employeeB的值也改变了。这就是浅拷贝。浅拷贝只拷贝对象引用本身，而不去拷贝引用的成员属性。从这个层次上来说，深clone并不是特别困难，简单地说，就是创建好对象，再设置一些成员属性。

       接下来看看java的clone技术是怎么实现深浅拷贝的。

     java中跟克隆有关的两个类分别是Cloneable接口和Object类中的clone方法，通过两者的协作来实现克隆。首先看一下java api doc中关于Cloneable接口和Object类中的clone方法的描述：

                      java.lang.Cloneable 接口,

          此类实现了 Cloneable 接口，以指示 Object.clone() 方法可以合法地对该类实例进行按字段复制。如果在没有实现 Cloneable 接口的实例上调用 Object 的 clone 方法，则会导致抛出 CloneNotSupportedException异常。 按照惯例，实现此接口的类应该使用公共方法重写 Object.clone（它是受保护的）。请参阅 Object.clone()，以获得有关重写此方法的详细信息。 

       Cloneable接口没有任何方法，仅是个标志接口(tagging interface)，若要具有克隆能力，实现Cloneable接口的类必须重写从Object继承来的clone方法，并调用Object的clone方法，重写后的方法应为public 的。注意的是：java默认的clone（）方法是浅拷贝，若要拷贝基本类型外加String类型以外的类型，即聚合或组合类间关系的时候，就需要进行深拷贝了。此时，常用的解决方法是纵深clone，即克隆到基本类型，外加String类型以外，不能再克隆为止。如下述例子：

 

public class FamilyInfo implements Cloneable{
    public String address;
    public int memberNum;

    public FamilyInfo(String address, int memberNum) {
        super();
        this.address = address;
        this.memberNum = memberNum;
    }

    public FamilyInfo clone(){
        FamilyInfo familyInfo=null;
        try {
            familyInfo=(FamilyInfo) super.clone();
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return familyInfo;
    }
}

public class FamilyInfo implements Cloneable{
	public String address;
	public int memberNum;
	
	public FamilyInfo(String address, int memberNum) {
		super();
		this.address = address;
		this.memberNum = memberNum;
	}
	
	public FamilyInfo clone(){
		FamilyInfo familyInfo=null;
		try {
			familyInfo=(FamilyInfo) super.clone();
		} catch (CloneNotSupportedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		return familyInfo;
	}
}

public class Employee implements Cloneable{
    public String name;
    public FamilyInfo familyInfo;

    public Employee(String name, FamilyInfo familyInfo) {
        super();
        this.name = name;
        this.familyInfo = familyInfo;
    }

    public Employee clone(){
        Employee employee=null;
        try {
            employee=(Employee) super.clone();
            if(this.familyInfo!=null){
                employee.familyInfo=this.familyInfo.clone();
            }
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return employee;

    }

}

public class Employee implements Cloneable{
	public String name;
	public FamilyInfo familyInfo;
	
	public Employee(String name, FamilyInfo familyInfo) {
		super();
		this.name = name;
		this.familyInfo = familyInfo;
	}
	
	public Employee clone(){
		Employee employee=null;
		try {
			employee=(Employee) super.clone();
			if(this.familyInfo!=null){
				employee.familyInfo=this.familyInfo.clone();
			}
		} catch (CloneNotSupportedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		return employee;
		
	}
	
}

 

public class CloneTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        FamilyInfo familyInfoA=new FamilyInfo("No.1100 Lianhang Rd.",2);
        Employee employeeA=new Employee("Lily",familyInfoA);
        System.out.println("employeeA's address "+employeeA.familyInfo.address);
        Employee employeeB=employeeA.clone();
        System.out.println("employeeB's address "+employeeB.familyInfo.address);
        System.out.println("---------------------");
        employeeA.familyInfo.address="No.1588 Pulian Rd.";
        System.out.println("employeeA's address "+employeeA.familyInfo.address);
        System.out.println("employeeB's address "+employeeB.familyInfo.address);

    }
}

public class CloneTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		FamilyInfo familyInfoA=new FamilyInfo("No.1100 Lianhang Rd.",2);
		Employee employeeA=new Employee("Lily",familyInfoA);
		System.out.println("employeeA's address "+employeeA.familyInfo.address);
		Employee employeeB=employeeA.clone();
		System.out.println("employeeB's address "+employeeB.familyInfo.address);
		System.out.println("---------------------");
		employeeA.familyInfo.address="No.1588 Pulian Rd.";
		System.out.println("employeeA's address "+employeeA.familyInfo.address);
		System.out.println("employeeB's address "+employeeB.familyInfo.address);	

	}
}

 

 

输出结果为：

 

 

可以看到，深拷贝后，两个对象彼此独立，不受影响。