synchronized實現同步

利用synchronized關鍵字是最簡單的實現同步的方式，synchronized可以用於方法，代碼塊，synchronized可以保證在同一時刻，只有一個線程可以執行某個方法或某個代碼塊，同時synchronized可以保證一個線程的變化可見。也就是保證了臨界區某一時刻只能被一個線程訪問（被synchronized修飾的方法，或者代碼塊都是臨界區）。當一個線程進入了臨界區，其他試圖訪問該臨界區的線程將會被掛起，知道第一個線程執行完正在運行的方法。

synchronized的使用：

• 普通方法，鎖是當前對象的實例
• 靜態方法，鎖是當前類的class對象
• 同步方法塊，鎖是括號裏面的對象。

注意：
對於靜態同步方法存在這樣一種情況，因爲鎖是當前類的class對象，所以同一時間只能有一個線程訪問，但是對於其他的普通方法，其他線程也是可以訪問修改的，如果其他方法中對臨界區中的數據進行了修改，也會造成數據的不一致。

代碼示例講解，最簡單的例子是對一個數進行加減法。

普通方法

// add del 方法都是臨界區
public class Demo8 {

	public static void main(String[] args) {
		long number = 100;
		NumberData numberData = new NumberData(number);
		Thread thread1 = new Thread(new User(numberData, 10));
		Thread thread2 = new Thread(new User(numberData, -50));
		Thread thread3 = new Thread(new User(numberData, 10));
		Thread thread4 = new Thread(new User(numberData, -10));
		Thread thread5 = new Thread(new User(numberData, -40));
		Thread thread6 = new Thread(new User(numberData, 10));
		Thread thread7 = new Thread(new User(numberData, -10));
		
		thread1.start();
		thread2.start();
		thread3.start();
		thread4.start();
		thread5.start();
		thread6.start();
		thread7.start();
	}
	
	
	static class NumberData implements Serializable{
		private long number;

		public NumberData(long number) {
			this.number = number;
		}

		public long getNumber() {
			return number;
		}

		public void setNumber(long number) {
			this.number = number;
		}
		
		
		public synchronized void add(long num) {
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			number = number + num;
			System.out.println("=======add============    "+num+"  "+number);
		}
		
		public synchronized void del(long num) {
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			number = number + num;
			System.out.println("=======del============    "+num+"  "+number);
		}
	}
	
	static class User implements Runnable{

		private NumberData numberData;
		private long number;
		
		
		
		public long getNumber() {
			return number;
		}

		public void setNumber(long number) {
			this.number = number;
		}

		public NumberData getNumberData() {
			return numberData;
		}

		public void setNumberData(NumberData numberData) {
			this.numberData = numberData;
		}
		public User(NumberData numberData,long num) {
			super();
			this.numberData = numberData;
			this.number = num;
		}

		@Override
		public void run() {
			if (number > 0) {
				numberData.add(number);
			}else {
				numberData.del(number);
			}
		}
		
	}

}

同步代碼塊：

public class Demo8 {

	public static void main(String[] args) {
		long number = 100;
		NumberData numberData = new NumberData(number);
		Thread thread1 = new Thread(new User(numberData, 10));
		Thread thread2 = new Thread(new User(numberData, -50));
		Thread thread3 = new Thread(new User(numberData, 10));
		Thread thread4 = new Thread(new User(numberData, -10));
		Thread thread5 = new Thread(new User(numberData, -40));
		Thread thread6 = new Thread(new User(numberData, 10));
		Thread thread7 = new Thread(new User(numberData, -10));
		
		thread1.start();
		thread2.start();
		thread3.start();
		thread4.start();
		thread5.start();
		thread6.start();
		thread7.start();
	}
	
	
	static class NumberData implements Serializable{
		private long number;

		public NumberData(long number) {
			this.number = number;
		}

		public long getNumber() {
			return number;
		}

		public void setNumber(long number) {
			this.number = number;
		}
		
		
		public  void add(long num) {
			synchronized (this) {
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				number = number + num;
				System.out.println("=======add============    "+num+"  "+number);
			}
			
		}
		
		public void del(long num) {
			synchronized (this) {
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				number = number + num;
				System.out.println("=======del============    "+num+"  "+number);
			}
			
		}
	}
	
	static class User implements Runnable{

		private NumberData numberData;
		private long number;
		
		
		
		public long getNumber() {
			return number;
		}

		public void setNumber(long number) {
			this.number = number;
		}

		public NumberData getNumberData() {
			return numberData;
		}

		public void setNumberData(NumberData numberData) {
			this.numberData = numberData;
		}

		
		public User(NumberData numberData,long num) {
			super();
			this.numberData = numberData;
			this.number = num;
		}

		@Override
		public void run() {
			if (number > 0) {
				numberData.add(number);
			}else {
				numberData.del(number);
			}
		}
		
	}

}

或者設置一個公共的對象，作爲統一的一個鎖。

靜態同步方法：

public class Demo9 {
	public static void main(String[] args) {
		long number = 100;
		NumberData numberData = new NumberData(number);
		Thread thread1 = new Thread(new User(numberData, 10));
		Thread thread2 = new Thread(new User(numberData, -50));
		Thread thread3 = new Thread(new User(numberData, 10));
		Thread thread4 = new Thread(new User(numberData, -10));
		Thread thread5 = new Thread(new User(numberData, -40));
		Thread thread6 = new Thread(new User(numberData, 10));
		Thread thread7 = new Thread(new User(numberData, -10));
		Thread thread8 = new Thread() {
			@Override
			public void run() {
				NumberData.staticFun1();
			}
		};
		
		Thread thread9 = new Thread() {
			@Override
			public void run() {
				NumberData.staticFun2();
			}
		};
		
		thread1.start();
		thread2.start();
		thread3.start();
		thread4.start();
		thread5.start();
		thread6.start();
		thread7.start();
		
		thread8.start();
		thread9.start();
	}
	
	
	static class NumberData implements Serializable{
		private long number;

		public NumberData(long number) {
			this.number = number;
		}

		public long getNumber() {
			return number;
		}

		public void setNumber(long number) {
			this.number = number;
		}
		
		
		public  void add(long num) {
			synchronized (this) {
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				number = number + num;
				System.out.println("=======add============    "+num+"  "+number);
			}
			
		}
		
		public void del(long num) {
			synchronized (this) {
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				number = number + num;
				System.out.println("=======del============    "+num+"  "+number);
			}
			
		}
		
		public static synchronized void staticFun1() {
			for(int i=0;i<4;i++) {
				try {
					Thread.sleep(500);
					System.out.println("========staticFun1========");
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		}
		
         public static synchronized void staticFun2() {
        	 for(int i=0;i<4;i++) {
 				try {
 					Thread.sleep(500);
 					System.out.println("========staticFun2========");
 				} catch (InterruptedException e) {
 					e.printStackTrace();
 				}
 			}
		}
	}
	
	static class User implements Runnable{

		private NumberData numberData;
		private long number;
		
		
		
		public long getNumber() {
			return number;
		}

		public void setNumber(long number) {
			this.number = number;
		}

		public NumberData getNumberData() {
			return numberData;
		}

		public void setNumberData(NumberData numberData) {
			this.numberData = numberData;
		}

		
		public User(NumberData numberData,long num) {
			super();
			this.numberData = numberData;
			this.number = num;
		}

		@Override
		public void run() {
			if (number > 0) {
				numberData.add(number);
			}else {
				numberData.del(number);
			}
		}
		
	}

}

result：

=======add============    10  110
=======del============    -10  100
=======add============    10  110
=======del============    -40  70
========staticFun1========
=======del============    -10  60
=======add============    10  70
=======del============    -50  20
========staticFun1========
========staticFun1========
========staticFun1========
========staticFun2========
========staticFun2========
========staticFun2========
========staticFun2========

對於static方法synchronized鎖的對象是當前類的class對象，所以和普通的synchronized方法不存在互斥問題，可以同時執行，但對於同步static方法，同時只能有一個執行。