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;
        }
    });