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.
