参考答案
1. synchronized 和 java.util.concurrent.lock.Lock 之间的区别
- 实现层面不一样：synchronized 是 Java 关键字，JVM层面 实现加锁和释放锁；Lock 是一个接口，在代码层面实现加锁和释放锁。
- 是否自动释放锁：synchronized 在线程代码执行完或出现异常时自动释放锁；Lock 不会自动释放锁，需要在 finally {} 代码块显式地中释放锁。
- 是否一直等待：synchronized 会导致线程拿不到锁一直等待；Lock 可以设置尝试获取锁或者获取锁失败一定时间超时
- 获取锁成功是否可知：synchronized 无法得知是否获取锁成功；Lock 可以通过 tryLock 获得加锁是否成功
- 功能复杂性：synchronized 加锁可重入、不可中断、非公平；Lock 可重入、可判断、可公平和不公平、细分读写锁提高效率。
2. java.util.concurrent.lock.Lock 与 java.util.concurrent.lock.ReadWriteLock 之间的区别
- ReadWriteLock 定义了获取读锁和写锁的接口，读锁之间不互斥，非常适合读多、写少的场景。
3. 适用场景
- JDK 1.6 开始，对 synchronized 方式枷锁进行了优化，加入了偏向锁、轻量级锁和锁升级机制，性能得到了很大的提升（性能与 ReentrantLock 差不多）。
- 读多写少的情况下，考虑使用 ReadWriteLock。
synchronized、ReentrantLock、ReentrantReadWriteLock 启动 990 个线程读共享变量，10 个线程写共享变量。
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class TestLockPerformance {
public static Object obj = new Object();//用于 synchronized 获取锁
public static Lock lock = new ReentrantLock();//可重入锁
public static ReadWriteLock readWriteLock = new ReentrantReadWriteLock();//读写锁
public static final int READ = 0;
public static final int WRITE = 1;
// uuid，一个随机字符串
public static String uuid = UUID.randomUUID().toString();
public static void main(String[] args) throws InterruptedException {
// testSynchronized(1000);
testReentrantLock(1000);
// testReadWriteLock(1000);
}
public static void testSynchronized(int threadNum) throws InterruptedException {
long t1 = System.currentTimeMillis();
List<Thread> tList = new ArrayList<Thread>();
//启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid
for (int i = 0; i <threadNum; i++) {
Thread t;
if (i % 100 == 0) {
t = new Thread(new WorkerSynchronized(WRITE));
} else {
t = new Thread(new WorkerSynchronized(READ));
}
t.start();//启动线程
tList.add(t);
}
for (Thread t : tList) {
t.join();
}
long t2 = System.currentTimeMillis();
System.out.println("testSynchronized 耗时：" + (t2 - t1));
}
public static void testReentrantLock(int threadNum) throws InterruptedException {
long t1 = System.currentTimeMillis();
List<Thread> tList = new ArrayList<Thread>();
//启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid
for (int i = 0; i <threadNum; i++) {
Thread t;
if (i % 100 == 0) {
t = new Thread(new WorkerReentrantLock(WRITE));
} else {
t = new Thread(new WorkerReentrantLock(READ));
}
t.start();//启动线程
tList.add(t);
}
for (Thread t : tList) {
t.join();
}
long t2 = System.currentTimeMillis();
System.out.println("testReentrantLock 耗时：" + (t2 - t1));
}
public static void testReadWriteLock(int threadNUm) throws InterruptedException {
long t1 = System.currentTimeMillis();
List<Thread> tList = new ArrayList<Thread>();
//启动 threadNum - 向上取整 (0.01 * threadNum) 个线程读 uuid, 向上取整 (0.01 * threadNum) 个线程写 uuid
for (int i = 0; i <threadNUm; i++) {
Thread t;
if (i % 100 == 0) {
t = new Thread(new WorkerReadWriteLock(WRITE));
} else {
t = new Thread(new WorkerReadWriteLock(READ));
}
t.start();//启动线程
tList.add(t);
}
for (Thread t : tList) {
t.join();
}
long t2 = System.currentTimeMillis();
System.out.println("testReadWriteLock 耗时：" + (t2 - t1));
}
}
//工作线程，使用 synchronized 关键字加锁
class WorkerSynchronized implements Runnable {
//0-read;1-write
private int type;
WorkerSynchronized(int type) {
this.type = type;
}
//加锁读 TestLockPerformance.uuid 变量，并打印
private void read() {
synchronized (TestLockPerformance.obj) {
//休眠 20 毫秒，模拟任务执行耗时
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() +
" read uuid = " + TestLockPerformance.uuid);
}
}
//加锁写 TestLockPerformance.uuid 变量，并打印
private void write() {
synchronized (TestLockPerformance.obj) {
//休眠 20 毫秒，模拟任务执行耗时
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
TestLockPerformance.uuid = UUID.randomUUID().toString();
System.out.println(Thread.currentThread().getName() +
" write uuid = " + TestLockPerformance.uuid);
}
}
@Override
public void run() {
//type = 0,线程读 TestLockPerformance.uuid 变量
if (type == 0) {
read();
//type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量
} else {
write();
}
}
}
//工作线程，使用 ReentrantLock 加锁
class WorkerReentrantLock implements Runnable {
//0-read;1-write
private int type;
WorkerReentrantLock(int type) {
this.type = type;
}
//加锁读 TestLockPerformance.uuid 变量，并打印
private void read() {
TestLockPerformance.lock.lock();
try {
//休眠 20 毫秒，模拟任务执行耗时
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() +
" read uuid = " + TestLockPerformance.uuid);
} finally {
TestLockPerformance.lock.unlock();
}
}
//加锁写 TestLockPerformance.uuid 变量，并打印
private void write() {
TestLockPerformance.lock.lock();
try {
//休眠 20 毫秒，模拟任务执行耗时
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
TestLockPerformance.uuid = UUID.randomUUID().toString();
System.out.println(Thread.currentThread().getName() +
" write uuid = " + TestLockPerformance.uuid);
} finally {
TestLockPerformance.lock.unlock();
}
}
@Override
public void run() {
//type = 0,线程读 TestLockPerformance.uuid 变量
if (type == 0) {
read();
//type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量
} else {
write();
}
}
}
//工作线程，使用 ReentrantReadWriteLock 关键字加锁
class WorkerReadWriteLock implements Runnable {
//0-read;1-write
private int type;
WorkerReadWriteLock(int type) {
this.type = type;
}
//加锁读 TestLockPerformance.uuid 变量，并打印
private void read() {
TestLockPerformance.readWriteLock.readLock().lock();
try {
//休眠 20 毫秒，模拟任务执行耗时
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() +
" read uuid = " + TestLockPerformance.uuid);
} finally {
TestLockPerformance.readWriteLock.readLock().unlock();
}
}
//加锁写 TestLockPerformance.uuid 变量，并打印
private void write() {
TestLockPerformance.readWriteLock.writeLock().lock();
try {
//休眠 20 毫秒，模拟任务执行耗时
try {
Thread.sleep(20);
} catch (InterruptedException e) {
e.printStackTrace();
}
TestLockPerformance.uuid = UUID.randomUUID().toString();
System.out.println(Thread.currentThread().getName() +
" write uuid = " + TestLockPerformance.uuid);
} finally {
TestLockPerformance.readWriteLock.writeLock().unlock();
}
}
@Override
public void run() {
//type = 0,线程读 TestLockPerformance.uuid 变量
if (type == 0) {
read();
//type = 1,线程生成 uuid，写入 TestLockPerformance.uuid 变量
} else {
write();
}
}
}
调用测试方法 ：
testSynchronized(1000);
耗时：
Thread-0 write uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824 Thread-252 read uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824 Thread-251 read uuid = b7fb63d7-79cc-4cc0-84ed-5a9cd4de6824 . . . Thread-255 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92 Thread-254 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92 Thread-253 read uuid = d666bfe6-dc71-4df2-882a-d530a59d7e92 testSynchronized 耗时：22991
调用测试方法 ：
testReentrantLock(1000);
耗时：
Thread-0 write uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1 Thread-1 read uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1 Thread-485 read uuid = 4352eb13-d284-47ec-8caa-fc81d91d08e1 . . . Thread-997 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5 Thread-998 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5 Thread-999 read uuid = 9d7f0a78-5eb7-4506-9e98-e8e9a7a717a5 testReentrantLock 耗时：22935
调用测试方法 ：
testReadWriteLock(1000);
耗时：
Thread-0 write uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd Thread-277 read uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd Thread-278 read uuid = 81c13f80-fb19-4b27-9d21-2e99f8c8acbd . . . Thread-975 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7 Thread-971 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7 Thread-964 read uuid = 35be0359-1973-4a4f-85b7-918053d841f7 testReadWriteLock 耗时：543
通过耗时测试可以看出，使用 synchronized 和 ReentrantLock 耗时相近。
但是，由于 990 个线程读，10 个线程写，使用 ReentrantReadWriteLock 耗时 543 毫秒。
以上，是Java面试题【Java中的锁之间的区别是什么】的参考答案。
输出，是最好的学习方法。
欢迎在评论区留下你的问题、笔记或知识点补充~
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