如果你序列化一个单例类,然后两次重构它,那么你就会得到那个单例类的两个实例,除非你实现readResolve()方法,像下面这样:
例1 一个可序列化的单例类
Java代码
import org.apache.log4j.Logger;
public class Singleton implements java.io.Serializable {
public static Singleton INSTANCE = new Singleton();
protected Singleton() {
// Exists only to thwart instantiation.
}
private Object readResolve() {
return INSTANCE;
}
}
上面的单例类实现从readResolve()方法中返回一个唯一的实例;这样无论Singleton类何时被重构,它都只会返回那个相同的单例类实例。
例2测试了例1 的单例类
例2 测试一个可序列化的单例类
Java代码
import java.io.*;
import org.apache.log4j.Logger;
import junit.framework.Assert;
import junit.framework.TestCase;
public class SingletonTest extends TestCase {
private Singleton sone = null, stwo = null;
private static Logger logger = Logger.getRootLogger();
public SingletonTest(String name) {
super(name);
}
public void setUp() {
sone = Singleton.INSTANCE;
stwo = Singleton.INSTANCE;
}
public void testSerialize() {
logger.info("testing singleton serialization...");
writeSingleton();
Singleton s1 = readSingleton();
Singleton s2 = readSingleton();
Assert.assertEquals(true, s1 == s2);[/b]
}
private void writeSingleton() {
try {
FileOutputStream fos = new FileOutputStream("serializedSingleton");
ObjectOutputStream oos = new ObjectOutputStream(fos);
Singleton s = Singleton.INSTANCE;
oos.writeObject(Singleton.INSTANCE);
oos.flush();
}catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: " + se.getMessage());
}catch(IOException iox) {
logger.fatal("IO Exception: " + iox.getMessage());
}
}
private Singleton readSingleton() {
Singleton s = null;
try {
FileInputStream fis = new FileInputStream("serializedSingleton");
ObjectInputStream ois = new ObjectInputStream(fis);
s = (Singleton)ois.readObject();
}catch(ClassNotFoundException cnf) {
logger.fatal("Class Not Found Exception: " + cnf.getMessage());
}catch(NotSerializableException se) {
logger.fatal("Not Serializable Exception: " + se.getMessage());
}catch(IOException iox) {
logger.fatal("IO Exception: " + iox.getMessage());
}
return s;
}
public void testUnique() {
logger.info("testing singleton uniqueness...");
Singleton another = new Singleton();
logger.info("checking singletons for equality");
Assert.assertEquals(true, sone == stwo);
}
}
前面这个测试案例序列化例1 中的单例类,并且两次重构它。然后这个测试案例检查看是否被重构的单例类实例是同一个对象。下面是测试案例的输出:
Java代码
Buildfile: build.xml
init:
[echo] Build 20030422 (22-04-2003 11:32)
compile:
run-test-text:
[java] .INFO main: testing singleton serialization...
[java] .INFO main: testing singleton uniqueness...
[java] INFO main: checking singletons for equality
[java] Time: 0.1
[java] OK (2 tests)
单例模式结束语
单例模式简单却容易让人迷惑,特别是对于Java的开发者来说。在这篇文章中,作者演示了Java开发者在顾及多线程、类载入器和序列化情况如何实现单例模式。作者也展示了你怎样才能实现一个单例类的注册表,以便能够在运行期指定单例类。