Java多執行緒面試題小結

阿新 • • 發佈：2022-03-07

1.執行緒狀態

下面是JDK中定義的執行緒狀態

public enum State {
        /**
         * Thread state for a thread which has not yet started.
         */
        NEW,

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
          
*/
        RUNNABLE,

        /**
         * Thread state for a thread blocked waiting for a monitor lock.
         * A thread in the blocked state is waiting for a monitor lock
         * to enter a synchronized block/method or
         * reenter a synchronized block/method after calling
         * {@link 
 Object#wait() Object.wait}.
         */
        BLOCKED,

        /**
         * Thread state for a waiting thread.
         * A thread is in the waiting state due to calling one of the
         * following methods:
         * <ul>
         *   <li>{@link Object#wait() Object.wait} with no timeout</li>
         *   <li>{ 
@link #join() Thread.join} with no timeout</li>
         *   <li>{@link LockSupport#park() LockSupport.park}</li>
         * </ul>
         *
         * <p>A thread in the waiting state is waiting for another thread to
         * perform a particular action.
         *
         * For example, a thread that has called <tt>Object.wait()</tt>
         * on an object is waiting for another thread to call
         * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
         * that object. A thread that has called <tt>Thread.join()</tt>
         * is waiting for a specified thread to terminate.
         */
        WAITING,

        /**
         * Thread state for a waiting thread with a specified waiting time.
         * A thread is in the timed waiting state due to calling one of
         * the following methods with a specified positive waiting time:
         * <ul>
         *   <li>{@link #sleep Thread.sleep}</li>
         *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
         *   <li>{@link #join(long) Thread.join} with timeout</li>
         *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
         *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
         * </ul>
         */
        TIMED_WAITING,

        /**
         * Thread state for a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }

　　把其中的Runnable狀態時是否獲取到cpu時間片，可為Runnable和Running討論，就有下面一張圖

2.建立執行緒

　　1.繼承Thread類 2.實現Runnable介面 3.實現Callable介面（有返回值，可丟擲異常） 4執行緒池executors

3.有關執行緒的幾個方法

　　1.sleep　　睡眠，不釋放鎖。

　　2.wait　　進入等待佇列，釋放鎖。

　　3.notify　　喚醒等待佇列的某個執行緒。和wait一樣必須在synchronized程式碼塊中使用。

　　4.yield　　讓步，由立即進入就緒狀態。

　　5.await和signal condition類的方法，對應wait和notify（Object類），一個condition相當於一個鎖池。

4.手寫死鎖

public class DeadLock {
    public static void main(String[] args) {
        Thread thread = new DeadLockThread("Thread1");
        Thread thread2 = new DeadLockThread("Thread2");
        thread.start();
        thread2.start();
    }
}
class DeadLockThread extends Thread{
    private static final Object A = new Object();
    private static final Object B = new Object();
    public DeadLockThread(String name){
        setName(name);
    }
    @Override
    public void run() {
        if ("Thread1".equals(getName())){
            synchronized (A){
                try {
                    sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Thread1StartAndGetLockA");
                synchronized (B){
                    System.out.println("Thread1StartAndGetLockB");
                }
            }
        }
        if ("Thread2".equals(getName())){
            synchronized (B){
                System.out.println("Thread2StartAndGetLockB");
                synchronized (A){
                    System.out.println("Thread2StartAndGetLockA");
                }
            }
        }
    }
}