靜態內部類：

package thread.singleton;

public class StaticInnerClassSingleton {
	private static class Singleton{
		private static Singleton singleton = new Singleton();
	}
	public Singleton getInstance(){
		return Singleton.singleton;
	}
}

package thread.singleton;


public class SynchronizedLazySingleton {
	private static SynchronizedLazySingleton lazySingleton;
	
	private SynchronizedLazySingleton(){}
	
	public static SynchronizedLazySingleton getInstance(){
		//效率提高，避免後續執行緒都嘗試獲取鎖照成鎖競爭
		if(lazySingleton == null){
			try {
				//模擬共享物件初始化時間
				//TimeUnit.SECONDS.sleep(1000);
				Thread.sleep(1000);
				synchronized (SynchronizedLazySingleton.class) {
					//在進入sychronzied方法體後，如果不進行if判斷，會引發執行緒安全問題
					if(lazySingleton == null){
						lazySingleton = new SynchronizedLazySingleton();
					}
				}
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
		return lazySingleton;
	}
	public static void main(String[] args) {
		new Thread(new Runnable() {
			
			@Override
			public void run() {
				System.out.println(SynchronizedLazySingleton.getInstance().hashCode());
				
			}
		}).start();
		new Thread(new Runnable() {
				
				@Override
				public void run() {
					System.out.println(SynchronizedLazySingleton.getInstance().hashCode());
					
				}
			}).start();
		new Thread(new Runnable() {
			
			@Override
			public void run() {
				System.out.println(SynchronizedLazySingleton.getInstance().hashCode());
				
			}
		}).start();
		new Thread(new Runnable() {
			
			@Override
			public void run() {
				System.out.println(SynchronizedLazySingleton.getInstance().hashCode());
				
			}
		}).start();
	}
}