五种单件模式之Singleton的实现方法详解

package singletonpattern;
public class Singleton1 {
 private static Singleton1 uniqueInstance;
 private Singleton1() {
 }
 public static Singleton1 getInstance() {
  if (uniqueInstance == null) {
   uniqueInstance = new Singleton1();
  }
  return uniqueInstance;
 }
}

package singletonpattern;
public class Singleton2 {
 private static Singleton2 uniqueInstance;
 private Singleton2() {
 }
 public static synchronized Singleton2 getInstance() {
  if (uniqueInstance == null) {
   uniqueInstance = new Singleton2();
  }
  return uniqueInstance;
 }
}

package singletonpattern;
public class Singleton3 {
 private static Singleton3 uniqueInstance = new Singleton3();
 private Singleton3() {
 }
 public static Singleton3 getInstance() {
  return uniqueInstance;
 }
}

public class Singleton5 {

 private Singleton5(){

 }

 public static Singleton5 getInstance(){
  return Nested.instance;
 }

 static class Nested{
  static Singleton5 instance = new Singleton5();
 }
}

package singletonpattern;
public class Singleton4 {
 private volatile static Singleton4 uniqueInstance;
 private Singleton4() {
 }
 public static Singleton4 getInstance() {
  if (uniqueInstance == null) {
   synchronized (Singleton4.class) {
    if (uniqueInstance == null) {
     uniqueInstance = new Singleton4();
    }
   }
  }
  return uniqueInstance;
 }
}