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锁如何使用？有什么注意事项？

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Java 中常见的锁有

synchronized 有 3种用法

package constxiong.concurrency.a18;import java.util.ArrayList;import java.util.List;/** * 测试 synchronized 普通方法 * @author ConstXiong * @date 2019-09-19 10:49:46 */public class TestSynchronizedNormalMethod {private int count = 0;//private void add1000() {private synchronized void add1000() { //使用 synchronized 修饰 add100 方法，即可获得正确的值 30000for (int i = 0; i <1000; i++) {count++;}}//启动 30 个线程，每个线程 对 TestSynchronized 对象的 count 属性加 1000private void test() throws InterruptedException {List<Thread> threads = new ArrayList<Thread>(10);for (int i = 0; i <30; i++) {Thread t =new Thread(() -> {add1000();});t.start();threads.add(t);}//等待所有线程执行完毕for (Thread t : threads) {t.join();}//打印 count 的值System.out.println(count);}public static void main(String[] args) throws InterruptedException {//创建 TestSynchronizedNormalMethod 对象，调用 test 方法new TestSynchronizedNormalMethod().test();}}
package constxiong.concurrency.a18;import java.util.ArrayList;import java.util.List;/** * 测试 synchronized 静态方法 * @author ConstXiong * @date 2019-09-19 10:49:46 */public class TestSynchronizedStaticMethod {private static int count = 0;private static void add1000() {//private synchronized static void add1000() { //使用 synchronized 修饰 add100 方法，即可获得正确的值 30000for (int i = 0; i <1000; i++) {count++;}}public static void main(String[] args) throws InterruptedException {//启动 30 个线程，每个线程 对 TestSynchronized 对象的 count 属性加 1000List<Thread> threads = new ArrayList<Thread>(10);for (int i = 0; i <30; i++) {Thread t =new Thread(() -> {add1000();});t.start();threads.add(t);}//等待所有线程执行完毕for (Thread t : threads) {t.join();}//打印 count 的值System.out.println(count);}}
package constxiong.concurrency.a18;import java.util.ArrayList;import java.util.List;/** * 测试 synchronized 代码块 * @author ConstXiong * @date 2019-09-19 10:49:46 */public class TestSynchronizedCodeBlock {private int count = 0;//锁定的对象private final Object obj = new Object();private void add1000() {//执行下面的加 1000 的操作，都需要获取 obj 这个对象的锁synchronized (obj) {for (int i = 0; i <1000; i++) {count++;}}}//启动 30 个线程，每个线程 对 TestSynchronized 对象的 count 属性加 1000private void test() throws InterruptedException {List<Thread> threads = new ArrayList<Thread>(10);for (int i = 0; i <30; i++) {Thread t =new Thread(() -> {add1000();});t.start();threads.add(t);}//等待所有线程执行完毕for (Thread t : threads) {t.join();}//打印 count 的值System.out.println(count);}public static void main(String[] args) throws InterruptedException {//创建 TestSynchronizedNormalMethod 对象，调用 test 方法new TestSynchronizedCodeBlock().test();}}

可重入锁 java.util.concurrent.lock.ReentrantLock 的使用示例

package constxiong.concurrency.a18;import java.util.ArrayList;import java.util.List;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/** * 测试 ReentrantLock  * @author ConstXiong * @date 2019-09-19 11:26:50 */public class TestReentrantLock {private int count = 0;private final Lock lock = new ReentrantLock();private void add1000() {lock.lock();try {for (int i = 0; i <1000; i++) {count++;}} finally {lock.unlock();}}//启动 30 个线程，每个线程 对 TestSynchronized 对象的 count 属性加 1000private void test() throws InterruptedException {List<Thread> threads = new ArrayList<Thread>(10);for (int i = 0; i <30; i++) {Thread t =new Thread(() -> {add1000();});t.start();threads.add(t);}//等待所有线程执行完毕for (Thread t : threads) {t.join();}//打印 count 的值System.out.println(count);}public static void main(String[] args) throws InterruptedException {//创建 TestReentrantLock 对象，调用 test 方法new TestReentrantLock().test();}}

可重复读写锁 java.util.concurrent.lock.ReentrantReadWriteLock 的使用示例

package constxiong.concurrency.a18;import java.util.HashMap;import java.util.Map;import java.util.Random;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock; /** * 测试可重入读写锁 ReentrantReadWriteLock * @author ConstXiong * @date 2019-09-19 11:36:19 */public class TestReentrantReadWriteLock {//存储 key value 的 mapprivate Map<String, Object> map = new HashMap<String, Object>();//读写锁private final ReadWriteLock lock = new ReentrantReadWriteLock();/** * 根据 key 获取 value * @param key */public Object get(String key) {Object value = null;lock.readLock().lock();try {Thread.sleep(50L);value = map.get(key);} catch (InterruptedException e) {e.printStackTrace();} finally {lock.readLock().unlock();}return value; }/** * 设置key-value * @param key */public void set(String key, Object value) {lock.writeLock().lock();try {Thread.sleep(50L);map.put(key, value);} catch (InterruptedException e) {e.printStackTrace();} finally {lock.writeLock().unlock();}} //测试5个线程读数据，5个线程写数据public static void main(String[] args) {//创建测试可重入读写锁 TestReentrantReadWriteLock 对象TestReentrantReadWriteLock test = new TestReentrantReadWriteLock();String key = "lock";//存入 map 中的 keyRandom r = new Random();//生成随机数作为 valuefor (int i = 0; i <5; i++) {//5 个线程读 map 中 key 的 valuenew Thread(() -> {for (int j = 0; j <10; j++) {System.out.println(Thread.currentThread().getName() + " read value=" + test.get(key));}}).start();//5 个线程写 map 中 key 的 valuenew Thread(() -> {for (int j = 0; j <10; j++) {int value = r.nextInt(1000);test.set(key, value);System.out.println(Thread.currentThread().getName() + " write value=" + value);}}).start();}}}

锁的使用注意事项

Integer i = 100;

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