JAVA 多线程面试题

参考Java线程的6种状态及切换(透彻讲解)

线程池参数 老八股文了。俩个时间参数是 线程空闲回收时间，回收时候用的。
execute 提交时候的流程 老八股文了。
线程回收条件

while (task != null || (task = getTask()) != null) {
//task  为null 会去 拿线程执行，拿不到就回收。
}
拿不到的条件。
1、队列空。线程池停了等情况
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }
2、 （工作线程大于最小线程 || 超过空闲时间）&&（工作线程大于1 || 队列空）
工作线程大于1 是应为自己停了怎么也得有个在运行的，队列空就不需要了
 int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

可以取个巧直接用synchronized wait notifyAll

Object o = new Object();
        AtomicInteger count = new AtomicInteger(0);
        new Thread(new P("a", o, count),"aaa").start();
        new Thread(new P("b", o, count),"bbb").start();

static class P implements Runnable{
        String info;
        final Object lock;
        AtomicInteger count;

        public P(String info, Object lock, AtomicInteger count) {
        //省略
        }

        @SneakyThrows
        @Override
        public void run() {
            synchronized (lock){
                while (this.count.get() < 20){
                   log.info(info + "" +count.incrementAndGet());
                    lock.notifyAll();
//                    lock.wait();
                    lock.wait(1000*20);
                    log.info("100毫秒");
                }    
            }
        }
    }

用ReentrantLock的Condition

public static void main(String[] args){
        AtomicInteger count = new AtomicInteger(0);

        ReentrantLock lock = new ReentrantLock();
        // 声明互相等待的条件
        Condition condition1 = lock.newCondition();
        Condition condition2 = lock.newCondition();
        Condition condition3 = lock.newCondition();
        new Thread(new P2(lock,condition1,condition2,"a",  count),"aaa").start();
        new Thread(new P2(lock,condition2,condition3,"b",  count),"bbb").start();
        new Thread(new P2(lock,condition3,condition1,"c",  count),"ccc").start();
        lock.lock();
        condition1.signal();
        lock.unlock();
    }
    
static class P2 implements Runnable{
        public P2(ReentrantLock lock,Condition condition, Condition conditionNext, String info, AtomicInteger count) {
            this.lock = lock;
            this.condition = condition;
            this.conditionNext = conditionNext;
            this.info = info;
            this.count = count;
        }
        ReentrantLock lock;
        Condition condition;
        Condition conditionNext;
        String info;
        AtomicInteger count;
        @SneakyThrows
        @Override
        public void run() {
                while (this.count.get() < 20) {
                    lock.lock();
                    condition.await();
                    log.info(info + "" + count.incrementAndGet());
                    conditionNext.signal();
                    lock.unlock();
                }    
                
            }
            
        }