工作中涉及到了一个普通的http服务,接收http请求,业务中需要保证同一个user只能同时有一个线程在处理,需要手动加锁.
由于只部署单个实例,所以没有使用分布式锁,第一想到的还是一个synchronized,使用synchronized对user加锁.测试代码如下:
/**
* @author: hman
* @date 2019-04-25
* @desc: //TODO
*/
public class TestSynchronized {
public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(10);
Random random = new Random();
//10个线程
for (int i = 0; i < 10; i++) {
executorService.execute(() -> {
//使用随机数模拟user并发
test(random.nextInt(3));
});
}
}
private static void test(int a) {
//随机字符串+数字
String s = "asdfasdf";
String lock = s + a;
synchronized (lock) {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(lock);
}
}
}
打印出来的结果
asdfasdf0
asdfasdf0
asdfasdf2
asdfasdf1
asdfasdf1
asdfasdf0
asdfasdf0
asdfasdf2
asdfasdf2
asdfasdf0
明显是没锁到.
在网上查了查原因,发现synchronized比较锁是使用的==,比较的是内存地址,因为每次都会new一个String,所以内存肯定不同.
解决思路:
1.改成小于-127->127的Integer,因为在这个范围的Integer是直接从常量池中取得.建议直接使用key取hash取模128即可
2.也是最简单最有效的方法.使用String的intern方法.
/**
* Returns a canonical representation for the string object.
* <p>
* A pool of strings, initially empty, is maintained privately by the
* class {@code String}.
* <p>
* When the intern method is invoked, if the pool already contains a
* string equal to this {@code String} object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this {@code String} object is added to the
* pool and a reference to this {@code String} object is returned.
* <p>
* It follows that for any two strings {@code s} and {@code t},
* {@code s.intern() == t.intern()} is {@code true}
* if and only if {@code s.equals(t)} is {@code true}.
* <p>
* All literal strings and string-valued constant expressions are
* interned. String literals are defined in section 3.10.5 of the
* <cite>The Java™ Language Specification</cite>.
*
* @return a string that has the same contents as this string, but is
* guaranteed to be from a pool of unique strings.
*/
public native String intern();
简单介绍,这个intern方法在newString的时候会先去字符串常量池中判断有没有,如果有就用,没有的话就new然后放进去,这样可以完美解决这个问题
代码修改成:
String lock = s + a;
synchronized (lock.intern()) {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
解决
