java | synchronized 锁消除

锁优化时 JIT 「即时编译器」自带的。打成 jar 包后，自动启动。

package com.redisc;

import lombok.Data;
import lombok.extern.slf4j.Slf4j;
import org.openjdk.jol.info.ClassLayout;

import java.util.concurrent.TimeUnit;

@Data
class User {
}

@Slf4j(topic = "c.Test")
public class Test {
    public static void main(String[] args) throws Exception {
        User user1 = new User();
        int t = 0;
        long start = System.nanoTime();
        for (int i = 0; i < 10000000; i++) {
            t++;
        }
        log.debug("{}", System.nanoTime() - start);
        start = System.nanoTime();
        for (int i = 0; i < 10000000; i++) {
            synchronized (user1) {

t++;
            }
        }
        log.debug("{}", System.nanoTime() - start);
    }
}

打成 jar 包

13:15:07.528 [main] DEBUG c.Test - 38901618
13:15:07.553 [main] DEBUG c.Test - 21990764

为什么后面加锁反而更快了，我个人认为是 JIT 在初次运行的时候，还没有优化好，等运行到加锁代码的时候，已经优化好了。

测试一下，代码改成，并打包

package com.redisc;

import lombok.Data;
import lombok.extern.slf4j.Slf4j;
import org.openjdk.jol.info.ClassLayout;

import java.util.concurrent.TimeUnit;

@Data
class User {
}

@Slf4j(topic = "c.Test")
public class Test {
    public static void main(String[] args) throws Exception {
        User user1 = new User();
        int t = 0;
        long start = System.nanoTime();
        for (int i = 0; i < 10000000; i++) {
            t++;
        }
        log.debug("{}", System.nanoTime() - start);
        start = System.nanoTime();
        for (int i = 0; i < 10000000; i++) {
            synchronized (user1) {

t++;
            }
        }
        log.debug("{}", System.nanoTime() - start);
        start = System.nanoTime();
        for (int i = 0; i < 10000000; i++) {
            synchronized (user1) {

t++;
            }
        }
        log.debug("{}", System.nanoTime() - start);
    }
}

13:17:13.907 [main] DEBUG c.Test - 39486256
13:17:13.931 [main] DEBUG c.Test - 21059873
13:17:13.953 [main] DEBUG c.Test - 21436095

在 IDEA 中运行

13:15:44.900 [main] DEBUG c.Test - 2505509
13:15:45.084 [main] DEBUG c.Test - 179604356