1 2 3 4 5 6 7 8 9 | @ThreadSafe public class SafeLazyInitialization { private static Resource resource; public synchronized static Resource getInstance() { if (resource == null) resource = new Resource(); return resource; } } |
另外一种写法：
1 2 3 4 5 | @ThreadSafe public class EagerInitialization { private static Resource resource = new Resource(); public static Resource getResource() { return resource; } } |
延迟初始化的写法：
1 2 3 4 5 6 7 8 9 | @ThreadSafe public class ResourceFactory { private static class ResourceHolder { public static Resource resource = new Resource(); } public static Resource getResource() { return ResourceHolder.resource ; } } |
二次检查锁定/Double Checked Locking的写法（反模式）
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | public class SingletonDemo { private static volatile SingletonDemo instance = null;//注意需要volatile private SingletonDemo() { } public static SingletonDemo getInstance() { if (instance == null) { //二次检查，比直接用独占锁效率高 synchronized (SingletonDemo .class){ if (instance == null) { instance = new SingletonDemo (); } } } return instance; } } |