容器单例
我们先看代码吧
public class ContainerSingleton {
private ContainerSingleton(){
}
private static Map<String,Object> singletonMap = new HashMap<String,Object>();
public static void putInstance(String key,Object instance){
if(StringUtils.isNotBlank(key) && instance != null){
if(!singletonMap.containsKey(key)){
singletonMap.put(key,instance);
}
}
}
public static Object getInstance(String key){
return singletonMap.get(key);
}
}
这种方式实现的单例是线程不安全的。如果需要线程安全的可以使用HashTable但是HashTable每次存取都会加上同步锁，性能损耗比较严重。或者使用ConcurrentHashMap。
ThreadLocal “单例“
这个单例严格意义上讲并不完全算是单例，它只能算在单个线程中的单例，也就是在同一个线程中的它是单例的。我们直接看代码吧。
public class ThreadLocalInstance {
private static final ThreadLocal<ThreadLocalInstance> threadLocalInstanceThreadLocal
= new ThreadLocal<ThreadLocalInstance>(){
@Override
protected ThreadLocalInstance initialValue() {
return new ThreadLocalInstance();
}
};
private ThreadLocalInstance(){
}
public static ThreadLocalInstance getInstance(){
return threadLocalInstanceThreadLocal.get();
}
}
然后我们写个测试类测试一下。
我们定义一个线程T，在这个线程里面获取这个单例对象。
public class T implements Runnable {
@Override
public void run() {
ThreadLocalInstance instance = ThreadLocalInstance.getInstance();
System.out.println(Thread.currentThread().getName()+" "+instance);
}
}
然后是测试类包含主函数。
public class SingletonTest {
public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
System.out.println("main thread"+ThreadLocalInstance.getInstance());
System.out.println("main thread"+ThreadLocalInstance.getInstance());
System.out.println("main thread"+ThreadLocalInstance.getInstance());
System.out.println("main thread"+ThreadLocalInstance.getInstance());
System.out.println("main thread"+ThreadLocalInstance.getInstance());
System.out.println("main thread"+ThreadLocalInstance.getInstance());
Thread t1 = new Thread(new T());
Thread t2 = new Thread(new T());
t1.start();
t2.start();
System.out.println("program end");
}
我们再看一下运行结果。
main threadcom.design.pattern.creational.singleton.ThreadLocalInstance@12edcd21
main threadcom.design.pattern.creational.singleton.ThreadLocalInstance@12edcd21
main threadcom.design.pattern.creational.singleton.ThreadLocalInstance@12edcd21
main threadcom.design.pattern.creational.singleton.ThreadLocalInstance@12edcd21
main threadcom.design.pattern.creational.singleton.ThreadLocalInstance@12edcd21
main threadcom.design.pattern.creational.singleton.ThreadLocalInstance@12edcd21
program end
Thread-0 com.design.pattern.creational.singleton.ThreadLocalInstance@3ec28870
Thread-1 com.design.pattern.creational.singleton.ThreadLocalInstance@7a56389b
从结果我们可以看出这个单例之算是个伪单例，只能在一个线程里面实现单例。