java单例的几种实现方法：

方式1：

public class Something {

    private Something() {}

    private static class LazyHolder {

        private static final Something INSTANCE = new Something();

    }

    public static Something getInstance() {

        return LazyHolder.INSTANCE;

    }

}

方式2：

public class Singleton {

    private static final Singleton INSTANCE = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {

        return INSTANCE;

    }

}

方式3：

public class Singleton {

    private static final Singleton instance;

    static {

        try {

            instance = new Singleton();

        } catch (Exception e) {

            throw new RuntimeException("Darn, an error occurred!", e);

        }

    }

    public static Singleton getInstance() {

        return instance;

    }

    private Singleton() {

        // ...

    }

}

方式4：

public enum Singleton {

    INSTANCE;

    public void execute (String arg) {

        // Perform operation here

    }

}

方式5：

public class SingletonDemo {

    private static volatile SingletonDemo instance;

    private SingletonDemo() { }

    public static SingletonDemo getInstance() {

        if (instance == null ) {

            synchronized (SingletonDemo.class) {

                if (instance == null) {

                    instance = new SingletonDemo();

                }

            }

        }

        return instance;

    }

}

方式6：

使用apache commons lang: LazyInitializer

 public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {

     @Override

     protected ComplexObject initialize() {

         return new ComplexObject();

     }

 }

 // Create an instance of the lazy initializer

 ComplexObjectInitializer initializer = new ComplexObjectInitializer();

 ...

 // When the object is actually needed:

 ComplexObject cobj = initializer.get();

方式7：

使用guava：

 private static final Supplier<String> tokenSup = Suppliers.memoize(new Supplier<String>() {

        @Override

        public String get() {

        	//do some init

        	String result = xxx;

            return result;

        }

    });