JAVA序列化/反序列化与单例

单例设计类:

 

package com.test.singleton;

import java.io.IOException;
import java.io.ObjectStreamException;
import java.io.Serializable;


public class SingleTon implements Serializable{

	/**
	 * 
	 */
	private static final long serialVersionUID = 768457893059530646L;

	private SingleTon(){
		System.out.println("here...");
		//避免反射机制,导致的多例问题,通过反射机制仍然可以对私有构造函数进行操作
		if(instance != null){
			return;
		}
	}
	
	private static final SingleTon instance = new SingleTon();
	
	public static SingleTon getInstance(){
		return instance;
	}
	
	public void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException{
		//
	}
	/**
	 * 严格单例,确保remote instance不会干扰单例模式,避免在发序列化过程中对单例的影响.
	 * @return
	 * @throws ObjectStreamException
	 */
	public Object readResolve() throws ObjectStreamException{
		return instance;
	}
}

 

///流方式:
package com.test.singleton;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class SerializeCheckMain {

	/**
	* @param args
	*/
	public static void main(String[] args) throws Exception{
		// TODO Auto-generated method stub
		SingleTon s1 = SingleTon.getInstance();
		File file = new File("D:\\singleTome.txt");
		ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(file));
		oos.writeObject(s1);
		oos.close();
		ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file));
		SingleTon s2 = (SingleTon)ois.readObject();
		System.out.println("HashCode 1 : " + s1.hashCode());
		System.out.println("HashCode 2 : " + s2.hashCode());
		System.out.println("Equal : " + (s1 == s2));//打破单例
	}

}

 

 

 

//反射方式:
package com.test.singleton;

import java.lang.reflect.Constructor;

/**
 * @author liuguanqing
 *	反射机制,测试单例
 */
public class RefletCheckMain {

	/**
	* @param args
	*/
	public static void main(String[] args) throws Exception{
		SingleTon s1 = SingleTon.getInstance();
		Constructor[] constructors = SingleTon.class.getDeclaredConstructors();
		Constructor<SingleTon> c = constructors[0];
		c.setAccessible(true);
		SingleTon s2 = c.newInstance(null);
		System.out.println("HashCode 1 : " + s1.hashCode());
		System.out.println("HashCode 2 : " + s2.hashCode());
		System.out.println("Equal : " + (s1 == s2));//打破单例
	}

}

 

注意,序列化和反序列化,是java使用字节码技术生成对象,将不会执行构造器方法.

注意到在writeReplace和readResolve,我们可以严格控制singleton的对象,在同一个JVM中完完全全只有唯一的对象,控制不让singleton对象产生副本.

 

package com.test.main;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;

public class TestSerializable implements Serializable{

	/**
	 * 
	 */
	private static final long serialVersionUID = -1740221592194879964L;

	private Integer age;
	
	private String name;
	
	public TestSerializable(){
		System.out.println("Contrustor..");
	}
	
	public Integer getAge() {
		return age;
	}


	public void setAge(Integer age) {
		this.age = age;
	}


	public String getName() {
		return name;
	}


	public void setName(String name) {
		this.name = name;
	}
	
	private void writeObject(java.io.ObjectOutputStream out) throws IOException{
		System.out.println("writeObject");
		out.writeInt(this.age);
		out.writeInt(this.name.getBytes("utf8").length);
		out.write(this.name.getBytes("utf8"));
	}
	
	private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException{
		this.age = in.readInt();
		int chars = in.readInt();
		byte[] bytes = new byte[chars];
		in.readFully(bytes);
		this.name = new String(bytes,"utf8");
		System.out.println("readObject");
	}
	
	public Object readResolve() throws ObjectStreamException{
		System.out.println("readResolve");
		return this;
	}
	
	public Object writeReplace() throws ObjectStreamException{
		System.out.println("writeReplace");
		return this;
	}


	/**
	 * @param args
	 */
	public static void main(String[] args) throws Exception{
		ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("D:\\sss.txt"));
		TestSerializable ss = new TestSerializable();
		ss.setAge(12);
		ss.setName("zhangsan");
		oos.writeObject(ss);
		oos.close();
		ObjectInputStream ois = new ObjectInputStream(new FileInputStream("D:\\sss.txt"));
		TestSerializable ss2 = (TestSerializable)ois.readObject();
		System.out.println(ss == ss2);
		System.out.println(ss2.getName());
		ois.close();
		//执行结果
		//Contrustor..
		//writeReplace
		//writeObject
		//readObject
		//readResolve
		//false
		//zhangsan
		
	}

}

 

//Externalizable接口是Serializable的子接口,不过此接口提供了两个特殊的扩展方法:
	@Override
	public void writeExternal(ObjectOutput out) throws IOException {
		System.out.println("writeEx");
		out.writeInt(50);
		
	}

	@Override
	public void readExternal(ObjectInput in) throws IOException,
			ClassNotFoundException {
		System.out.println("readEx");
		this.age = in.readInt();
	}

 

 

注意实现Externalizable接口的类,在发序列化时,将会执行构造函数,因为对于流操作而言,此对象是有明确类型的(Serializable接口是个标记接口).

而且,如果实现了writeExternal和readExternal,将不会在执行readObject和writeObject,因为此时这两个方法已经被"擦除".

 

对于java序列化和反序列化,被序列化的类中有关于serialVersionUID会带来一些问题;

1) 如果你调整了Class结构(比如新增/去除某个属性值,但是不能引入编译错误),但没有修改serialVersionUID;那么在反序列化和序列化时不会带来异常,只是可能导致部分属性在get时为null.

2) 如果你调整了Class结构,同时也修改了serialVersionUID;如果序列化和反序列双方没有保持uid一致的话,将会直接导致反序列化异常.(java.io.InvalidClassException)

3) 如果你没有显式的声明serialVersionUID,那么对于JVM而言,不同的class类结构(属性列表和方法列表)将会得出不同的uid;因此如果序列化双方不能保持一致的uid,仍然会带来问题.