? ? ? 經常看到網上的人說，可重入鎖一詞，但是總是沒怎么了解，到底什么是可重入鎖，一直是一個模糊的概念，下面來大致總結一下。

可重入鎖：指的是同一個線程多次試圖獲取它所占有的鎖，請求會成功。當釋放鎖的時候，直到重入次數清零，鎖才釋放完畢。

?? ? ?再之前的我的博客:java synchronized關鍵字的用法--對象鎖和類鎖

http://my.oschina.net/u/1540325/blog/682950

里面最后我貼了一個程序：

public class MultiLock {public synchronized void f1(int count, String threadName) {if (count-- > 0) {Print.print(threadName + ":f1() calling f2() with count " + count);f2(count, threadName);}}public synchronized void f2(int count, String threadName) {if (count-- > 0) {Print.print(threadName + ":f2() calling f1() with count " + count);f1(count, threadName);}}public static void main(String[] args) throws Exception {final MultiLock multiLock = new MultiLock();new Thread() {public void run() {String name = Thread.currentThread().getName();multiLock.f1(10, name);}}.start();new Thread() {public void run() {String name = Thread.currentThread().getName();multiLock.f2(10, name);}}.start();}
}
輸出結果如下：Thread-1:f2() calling f1() with count 9
Thread-1:f1() calling f2() with count 8
Thread-1:f2() calling f1() with count 7
Thread-1:f1() calling f2() with count 6
Thread-1:f2() calling f1() with count 5
Thread-1:f1() calling f2() with count 4
Thread-1:f2() calling f1() with count 3
Thread-1:f1() calling f2() with count 2
Thread-1:f2() calling f1() with count 1
Thread-1:f1() calling f2() with count 0
Thread-0:f1() calling f2() with count 9
Thread-0:f2() calling f1() with count 8
Thread-0:f1() calling f2() with count 7
Thread-0:f2() calling f1() with count 6
Thread-0:f1() calling f2() with count 5
Thread-0:f2() calling f1() with count 4
Thread-0:f1() calling f2() with count 3
Thread-0:f2() calling f1() with count 2
Thread-0:f1() calling f2() with count 1
Thread-0:f2() calling f1() with count 0

當時就覺得奇怪，在f1()里面調用了synchronized的f2(),當時只知道是可以，不知道具體原因，現在明白了，因為synchronized是可重入的鎖，所以同一個對象的synchronized的方法調用另外一個synchronized方法是不需要再次獲取鎖的。

?

synchronized和lock都是可重入的鎖，可以防止死鎖，相對于可重入的鎖有一種自旋鎖，后面再做研究。