Synchronized與ReentrantLock的區(qū)別

工作與面試中經(jīng)常會(huì)遇到Java常見的加鎖方法，本文著重介紹synchronized與ReentrantLock的區(qū)別。

① 底層實(shí)現(xiàn)上來說刺桃，synchronized 是JVM層面的鎖匈睁，是Java關(guān)鍵字，通過monitor對(duì)象來完成（monitorenter與monitorexit）蛋辈，對(duì)象只有在同步塊或同步方法中才能調(diào)用wait/notify方法属拾，ReentrantLock 是從jdk1.5以來（java.util.concurrent.locks.Lock）提供的API層面的鎖。

synchronized 的實(shí)現(xiàn)涉及到鎖的升級(jí)冷溶，具體為無鎖渐白、偏向鎖、自旋鎖逞频、向OS申請(qǐng)重量級(jí)鎖纯衍，ReentrantLock實(shí)現(xiàn)則是通過利用CAS（CompareAndSwap）自旋機(jī)制保證線程操作的原子性和volatile保證數(shù)據(jù)可見性以實(shí)現(xiàn)鎖的功能。

synchronized (new Object()){

}

new ReentrantLock();

使用javap -c對(duì)如上代碼進(jìn)行反編譯得到如下代碼：

image

② 是否可手動(dòng)釋放：

synchronized 不需要用戶去手動(dòng)釋放鎖苗胀，synchronized 代碼執(zhí)行完后系統(tǒng)會(huì)自動(dòng)讓線程釋放對(duì)鎖的占用襟诸； ReentrantLock則需要用戶去手動(dòng)釋放鎖瓦堵，如果沒有手動(dòng)釋放鎖，就可能導(dǎo)致死鎖現(xiàn)象励堡。一般通過lock()和unlock()方法配合try/finally語句塊來完成谷丸，使用釋放更加靈活。

private int number = 0;
    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    private AtomicInteger atomicInteger;

    public void increment() throws Exception {
        lock.lock();
        try {

            while (number != 0) {
                condition.await();
            }
            //do something
            number++;
            System.out.println(Thread.currentThread().getName() + "\t" + number);
            condition.signalAll();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }

    }

③ 是否可中斷

synchronized是不可中斷類型的鎖应结，除非加鎖的代碼中出現(xiàn)異撑偬郏或正常執(zhí)行完成； ReentrantLock則可以中斷鹅龄，可通過trylock(long timeout,TimeUnit unit)設(shè)置超時(shí)方法或者將lockInterruptibly()放到代碼塊中揩慕，調(diào)用interrupt方法進(jìn)行中斷。

public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }
public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

④ 是否公平鎖

synchronized為非公平鎖 ReentrantLock則即可以選公平鎖也可以選非公平鎖扮休，通過構(gòu)造方法new ReentrantLock時(shí)傳入boolean值進(jìn)行選擇迎卤，為空默認(rèn)false非公平鎖，true為公平鎖玷坠。

/**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

⑤ 鎖是否可綁定條件Condition

synchronized不能綁定蜗搔； ReentrantLock通過綁定Condition結(jié)合await()/singal()方法實(shí)現(xiàn)線程的精確喚醒，而不是像synchronized通過Object類的wait()/notify()/notifyAll()方法要么隨機(jī)喚醒一個(gè)線程要么喚醒全部線程八堡。

示例：用ReentrantLock綁定三個(gè)條件實(shí)現(xiàn)線程A打印一次1樟凄，線程B打印兩次2，線程C打印三次3
class Resource {
    private int number = 1;//A:1  B:2  C:3
    private Lock lock = new ReentrantLock();
    private Condition c1 = lock.newCondition();
    private Condition c2 = lock.newCondition();
    private Condition c3 = lock.newCondition();

    //1 判斷
    public void print1() {

        lock.lock();

        try {
            //判斷
            while (number != 1) {
                c1.await();
            }
            //2 do sth
            for (int i = 1; i < 2; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + number);
            }

            //3 通知
            number = 2;
            c2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    //1 判斷
    public void print2() {

        lock.lock();

        try {
            //判斷
            while (number != 2) {
                c2.await();
            }
            //2 do sth
            for (int i = 1; i < 3; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + number);
            }

            //3 通知
            number = 3;
            c3.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    //1 判斷
    public void print3() {

        lock.lock();

        try {
            //判斷
            while (number != 3) {
                c3.await();
            }
            //2 do sth
            for (int i = 1; i < 4; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + number);
            }

            //3 通知
            number = 1;
            c1.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

public static void main(String[] args) {

        Resource resource = new Resource();

        new Thread(()->{
            for (int i = 1; i <= 2; i++) {
                resource.print1();
            }
        },"A").start();

        new Thread(()->{
            for (int i = 1; i <= 2; i++) {
                resource.print2();
            }
        },"B").start();

        new Thread(()->{
            for (int i = 1; i <= 2; i++) {
                resource.print3();
            }
        },"C").start();

    }

輸出結(jié)果為：

A 1 B 2 B 2 C 3 C 3 C 3 A 1 B 2 B 2 C 3 C 3 C 3

⑥ 鎖的對(duì)象

synchronzied鎖的是對(duì)象兄渺，鎖是保存在對(duì)象頭里面的缝龄，根據(jù)對(duì)象頭數(shù)據(jù)來標(biāo)識(shí)是否有線程獲得鎖/爭搶鎖；ReentrantLock鎖的是線程挂谍，根據(jù)進(jìn)入的線程和int類型的state標(biāo)識(shí)鎖的獲得/爭搶