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.
原文链接:[http://wely.iteye.com/blog/2228826]