JAVA中Synchronized能否加锁字符串详解

synchronized (obj) {
    // 临界区
}

public class StringLockExample {
    public void doSomething(String lock) {
        synchronized (lock) {
            System.out.println(Thread.currentThread().getName() + " 获得了锁：" + lock);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ignored) {}
        }
    }
}

StringLockExample example = new StringLockExample();

Runnable task1 = () -> example.doSomething("LOCK");
Runnable task2 = () -> example.doSomething("LOCK");

new Thread(task1).start();
new Thread(task2).start();

String a = "LOCK";
String b = "LOCK";
System.out.println(a == b); // true

synchronized ("user_" + userId)

synchronized (("user_" + userId).intern())

private final ConcurrentHashMap<String, Object> lockMap = new ConcurrentHashMap<>();

public void doSomething(String key) {
    Object lock = lockMap.computeIfAbsent(key, k -> new Object());

    synchronized (lock) {
        // 临界区
    }
}

<!-- Maven -->
<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>32.1.1-jre</version>
</dependency>

import com.google.common.collect.Interner;
import com.google.common.collect.Interners;

public class GuavaInternerLock {
    private static final Interner<String> interner = Interners.newWeakInterner();

    public void doWork(String key) {
        String internedKey = interner.intern(key);
        synchronized (internedKey) {
            // 同样 key 的线程会同步执行
            System.out.println("Processing key: " + key);
        }
    }
}