Java多线程编程核心技术---单例模式与多线程

立即加载/饿汉模式

立即加载就是使用类的时候已经将对象创建完毕。

public class MyObject {
	//立即加载方式==饿汉模式
	private static MyObject myObject = new MyObject();
	private MyObject(){
	}
	public static MyObject getInstance(){
		//立即加载
		//缺点是不能有其他实例变量
		//getInstance()方法没有同步，有可能出现非线程安全问题
		return myObject;
	}
}

public class MyThread extends Thread {
	@Override
	public void run() {
		System.out.println(MyObject.getInstance().hashCode());
	}
}

public class Main {
	public static void main(String[] args) {
		MyThread t1 = new MyThread();
		MyThread t2 = new MyThread();
		MyThread t3 = new MyThread();
		t1.start();
		t2.start();
		t3.start();
	}
}

运行程序，控制台打印结果如下：

1795478472
1795478472
1795478472

控制台打印的hashCode是同一个值，说明对象是同一个，也就实现了立即加载型单例模式。

---

延迟加载/懒汉模式

延迟加载就是在调用get()方法时实例才被创建

public class MyObject {
	private static MyObject myObject;
	private MyObject(){
	}
	public static MyObject getInstance() {
		//延迟加载
		if (myObject != null) {
			return myObject;
		}
		myObject = new MyObject();
		return myObject;
	}
}

public class MyThread extends Thread {
	@Override
	public void run() {
		System.out.println(MyObject.getInstance().hashCode());
	}
}

public class Main {
	public static void main(String[] args) {
		MyThread t1 = new MyThread();
		t1.start();
	}
}

程序运行结果如下：

1396452035

此实验虽然取得一个对象的实例，但是如果是在多线程环境中，就会出现取出多个实例的情况。对以上代码中的main函数做如下修改：

public class Main {
	public static void main(String[] args) {
		MyThread t1 = new MyThread();
		MyThread t2 = new MyThread();
		MyThread t3 = new MyThread();
		t1.start();
		t2.start();
		t3.start();
	}
}

重新运行程序，控制台打印结果如下：

166471260
1795478472
1858758426

控制台打印了三个不同的hashCode，说明并没有实现单例模式。

---

延迟加载/懒汉模式 解决方案

public class MyObject {
	private static MyObject myObject;
	private MyObject(){
	}
	//整个方法上锁，效率较低
	synchronized public static MyObject getInstance() {
		//延迟加载
		if (myObject != null) {
			return myObject;
		}
		try {
		//模拟一些耗时操作
				Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		myObject = new MyObject();
		return myObject;
	}
}

重新运行程序，控制台将打印出三个一样的hashCode。

以上方法对整个方法加锁，效率比较低。对以上代码做如下修改：

public class MyObject {
	private static MyObject myObject;
	private MyObject(){
	}
	public static MyObject getInstance() {
		//延迟加载
		if (myObject != null) {
			return myObject;
		}
		try {
			//模拟一个耗时操作
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		synchronized (MyObject.class) {
			myObject = new MyObject();
		}
		return myObject;
	}
}

重新运行程序，控制台打印结果如下：

774088025
641502649
1367113803

以上代码虽然解决的效率问题，但是仍然没有保证只创建一个实例。继续对以上代码做如下修改：

public class MyObject {
	private static MyObject myObject;
	private MyObject(){
	}
	public static MyObject getInstance() {
		//延迟加载
		if (myObject != null) {
			return myObject;
		}
		try {
			//模拟一个耗时操作
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		synchronized (MyObject.class) {
			if (myObject == null) {
				myObject = new MyObject();
			}
		}
		return myObject;
	}
}

重新运行程序，控制台输入结果如下：

641502649
641502649
641502649

使用双重检查锁(DCL)功能，成功解决了懒汉模式遇到的多线程问题。DCL也是大多数多线程结合单例模式使用的解决方案。

---

使用静态内置类实现单例模式

public class MyObject {
	private static class MyObjectHandler{
		private static MyObject myObject = new MyObject();
	}
	private MyObject(){
		
	}
	public static MyObject getInstance () {
		return MyObjectHandler.myObject;
	}
}

运行程序，控制台输出结果如下：

774088025
774088025
774088025

---

序列化与反序列化的单例模式实现

public class MyObject implements Serializable {
	private static final long serialVersionUID = 1L;
	private static class MyObjectHandler{
		private static MyObject myObject = new MyObject();
	}
	private MyObject(){
		
	}
	public static MyObject getInstance () {
		return MyObjectHandler.myObject;
	}
}

public class SaveAndRead {
	public static void main(String[] args) {
		try {//序列化对象到磁盘
			MyObject myObject = MyObject.getInstance();
			FileOutputStream fileOutputStream = new FileOutputStream(new File("/Users/shangyidong/Downloads/myObjectFile.txt"));
			ObjectOutputStream objectOutput = new ObjectOutputStream(fileOutputStream);
			objectOutput.writeObject(myObject);
			objectOutput.close();
			fileOutputStream.close();
			System.out.println(myObject.hashCode());
		} catch (Exception e) {
			e.printStackTrace();
		}
		/****************/
		try {//从磁盘反序列化到对象
			FileInputStream fileInputStream = new FileInputStream(new File("/Users/shangyidong/Downloads/myObjectFile.txt"));
			ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
			MyObject myObject = (MyObject) objectInputStream.readObject();
			objectInputStream.close();
			fileInputStream.close();
			System.out.println(myObject.hashCode());
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

运行程序，控制台打印结果如下：

988487739
1878277481

控制台打印出的hashCode不同，存储到磁盘上的对象和从磁盘读取出来的对象并不是同一个对象。对以上代码做如下修改：

public class MyObject implements Serializable {
	private static final long serialVersionUID = 1L;
	private static class MyObjectHandler{
		private static MyObject myObject = new MyObject();
	}
	private MyObject(){
		
	}
	public static MyObject getInstance () {
		return MyObjectHandler.myObject;
	}
	protected Object readResolve(){
		System.out.println("readResolve invoked");
		return MyObjectHandler.myObject;
	}
}

重新运行程序，控制台打印结果如下：

1066557918
readResolve invoked
1066557918

反序列化时使用了ReadResolve()方法，存储到磁盘上的对象和从磁盘上取出来的对象是同一个对象。

---

使用static代码块实现单例模式

静态代码块中的代码在使用类的时候就已经执行了，所以可以应用静态代码块的这个特性来实现单例模式。

public class MyObject {
	private static MyObject myObject = null;
	private MyObject(){
		
	}
	static{
		myObject = new MyObject();
	}
	public static MyObject getInstance() {
		return myObject;
	}
}

public class MyThread extends Thread {
	@Override
	public void run() {
		for (int i = 0; i < 3; i++) {
			System.out.println(MyObject.getInstance().hashCode());
		}
	}
}

public class Main {
	public static void main(String[] args) {
		MyThread t1 = new MyThread();
		MyThread t2 = new MyThread();
		MyThread t3 = new MyThread();
		t1.start();
		t2.start();
		t3.start();
	}
}

运行程序，控制台输出结果如下：

708252873
708252873
708252873
708252873
708252873
708252873
708252873
708252873
708252873

---

使用enum枚举数据类型实现单例模式

枚举enum和静态代码块的特性相似，在使用枚举类时，构造方法会被自动调用，也可以实现单例设计模式。

public enum MyObject {
	connectionFactory;
	private Connection connection;
	private MyObject(){
		try {
			System.out.println("创建MyObject对象");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			String user = "root"; 
	        String password = "";
	        String driver = "com.mysql.jdbc.Driver";
	        Class.forName(driver);
	        connection = DriverManager.getConnection(url, user, password);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
	public Connection getConnection() {
		return connection;
	}
}

public class MyThread extends Thread {
	@Override
	public void run() {
		for (int i = 0; i < 3; i++) {
			System.out.println(MyObject.connectionFactory.getConnection().hashCode());
		}
	}
}

public class Main {
	public static void main(String[] args) {
		MyThread t1 = new MyThread();
		MyThread t2 = new MyThread();
		MyThread t3 = new MyThread();
		t1.start();
		t2.start();
		t3.start();
	}
}

运行程序，控制台打印结果如下：

创建MyObject对象
1253195928
1253195928
1253195928
1253195928
1253195928
1253195928
1253195928
1253195928
1253195928

---

完善使用enum枚举实现单例模式

上面的代码中将枚举类进行了暴露，违反了“职责单一原则”。下面进行改善：

public class MyObject {
	public enum MyEnumSingleton{
		connectionFactory;
		private Connection connection;
		private MyEnumSingleton(){
			try {
				System.out.println("创建MyObject对象");
				String url = "jdbc:mysql://127.0.0.1:3306/test";
				String user = "root"; 
		        String password = "";
		        String driver = "com.mysql.jdbc.Driver";
		        Class.forName(driver);
		        connection = DriverManager.getConnection(url, user, password);
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
		public Connection getConnection() {
			return connection;
		}	
	}
	public static Connection getConnection() {
		return MyEnumSingleton.connectionFactory.getConnection();
	}
}

public class MyThread extends Thread {
	@Override
	public void run() {
		for (int i = 0; i < 3; i++) {
			System.out.println(MyObject.getConnection().hashCode());
		}
	}
}

public class Main {
	public static void main(String[] args) {
		MyThread t1 = new MyThread();
		MyThread t2 = new MyThread();
		MyThread t3 = new MyThread();
		t1.start();
		t2.start();
		t3.start();
	}
}

程序运行结果如下：

创建MyObject对象
2039988480
2039988480
2039988480
2039988480
2039988480
2039988480
2039988480
2039988480
2039988480