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Depending on the usage, there are several "correct" answers.
Since java5 the best way to do it is to use an enum:
public enum Foo { INSTANCE; }
The Right Way to Implement a Serializable Singleton public enum Elvis { INSTANCE; private final String[] favoriteSongs = { "Hound Dog", "Heartbreak Hotel" }; public void printFavorites() { System.out.println(Arrays.toString(favoriteSongs)); } }
理解：JDK5之后，建议用用枚举类的方式实现单例的方式。
Pre java5, the most simple case is:
public final class Foo { private static final Foo INSTANCE = new Foo(); //加载Foo的时候对INSTANCE进行了实例化 private Foo() { if (INSTANCE != null) { throw new IllegalStateException("Already instantiated"); } } public static Foo getInstance() { return INSTANCE; } }
Let‘s go over the code. First, you want the class to be final. In this case, I‘ve used the final keyword to let the users know it is final. Then you need to make the constructor private to prevent users to create their own Foo. Throwing an exception from the constructor prevents users to use reflection to create a second Foo. Then you create a private static final Foo field to hold the only instance, and a public static Foo getInstance() method to return it. The Java specification makes sure that the constructor is only called when the class is first used.
When you have a very large object or heavy construction code AND also have other accessible static methods or fields that might be used before an instance is needed, then and only then you need to use lazy initialization.
You can use a private static class to load the instance. The code would then look like:
public final class Foo { private static class FooLoader { private static final Foo INSTANCE = new Foo(); } private Foo() { if (FooLoader.INSTANCE != null) { throw new IllegalStateException("Already instantiated"); } } public static Foo getInstance() { return FooLoader.INSTANCE; //调用getInstance()方法时对FooLoader类进行了加载并实例化了INSTANCE } }
Since the line private static final Foo INSTANCE = new Foo(); is only executed when the class FooLoader is actually used, this takes care of the lazy instantiation, and is it guaranteed to be thread safe.
When you also want to be able to serialize your object you need to make sure that deserialization won‘t create a copy.
public final class Foo implements Serializable { private static final long serialVersionUID = 1L; private static class FooLoader { private static final Foo INSTANCE = new Foo(); } private Foo() { if (FooLoader.INSTANCE != null) { throw new IllegalStateException("Already instantiated"); } } public static Foo getInstance() { return FooLoader.INSTANCE; } @SuppressWarnings("unused") private Foo readResolve() { return FooLoader.INSTANCE; } }
The method readResolve() will make sure the only instance will be returned, even when the object was serialized in a previous run of your program.
Item 3 - What is an efficient way to implement a singleton pattern in Java?
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原文地址：http://www.cnblogs.com/Guoyutian/p/5077524.html
