java面试题

Posted 2020-10-04

  最近看到一份面试题，觉得写的还不错，虽然作者只给出了题目没有答案，但是它既是一份面试题同样作者其实很好的总结了java程序员应该具备的技术体系，希望能够对初入java和迷茫不知学什么的同学以指引

原文链接

由于某些原因正打算换工作，所以试着整理解答，答案只是我的想法，如有不对之处欢迎大神批评指正，

由于题目太多，我会持续添加

　　1. Java线程的状态

 1  public enum State {
 2         /**
 3          * Thread state for a thread which has not yet started.
 4          */
 5         NEW,
 6 
 7         /**
 8          * Thread state for a runnable thread.  A thread in the runnable
 9          * state is executing in the Java virtual machine but it may
10          * be waiting for other resources from the operating system
11          * such as processor.
12          */
13         RUNNABLE,
14 
15         /**
16          * Thread state for a thread blocked waiting for a monitor lock.
17          * A thread in the blocked state is waiting for a monitor lock
18          * to enter a synchronized block/method or
19          * reenter a synchronized block/method after calling
20          * {@link Object#wait() Object.wait}.
21          */
22         BLOCKED,
23 
24         /**
25          * Thread state for a waiting thread.
26          * A thread is in the waiting state due to calling one of the
27          * following methods:
28          * <ul>
29          *   <li>{@link Object#wait() Object.wait} with no timeout</li>
30          *   <li>{@link #join() Thread.join} with no timeout</li>
31          *   <li>{@link LockSupport#park() LockSupport.park}</li>
32          * </ul>
33          *
34          * <p>A thread in the waiting state is waiting for another thread to
35          * perform a particular action.
36          *
37          * For example, a thread that has called <tt>Object.wait()</tt>
38          * on an object is waiting for another thread to call
39          * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
40          * that object. A thread that has called <tt>Thread.join()</tt>
41          * is waiting for a specified thread to terminate.
42          */
43         WAITING,
44 
45         /**
46          * Thread state for a waiting thread with a specified waiting time.
47          * A thread is in the timed waiting state due to calling one of
48          * the following methods with a specified positive waiting time:
49          * <ul>
50          *   <li>{@link #sleep Thread.sleep}</li>
51          *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
52          *   <li>{@link #join(long) Thread.join} with timeout</li>
53          *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
54          *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
55          * </ul>
56          */
57         TIMED_WAITING,
58 
59         /**
60          * Thread state for a terminated thread.
61          * The thread has completed execution.
62          */
63         TERMINATED;
64     }

　　1）New (新建)

　　当用new操作符创建一个新线程时，如 new Thread (runnable);此时该线程还没有开始

　　2）Runnable （可运行状态）

　　当线程有资格运行，但调度程序还没有把它选定为运行线程时线程所处的状态。当start()方法调用时，线程首先进入可运行状态。在线程运行之后或者从阻塞、等待或睡眠状态回来后，也返回到可运行状态。

　　3） Blocked （被阻塞）

　　当线程调用sleep()或者join()方法就会进入Blocked状态，但是要注意的是阻塞的线程是不释放当前所占有的系统资源，当sleep()结束或者join()等待其他线程来到，当前线程则进入Runnable状态等待JVM分配资源。

　　4）Waiting（等待）

　　如果当前线程调用wait()方法，则当前线程进入Time waiting但是这个时候当前线程会释放所占有的JVM资源，进入这个状态过后是不能自动唤醒的，必须调用notify()或者notifyAll()方法，线程进入Waiting。

　　5）Timed Waiting (计时等待)

　　当线程进入Runnable状态，但是还没有开始运行的时候，此时发现需要的资源处于同步状态synchronized，这个时候线程将会进入Time waiting,JVM会使用队列对这些线程进行控制，既先进行Time waiting的线程会先得到JVM资源进行执行进入Waiting

　　6）Terminated （被终止）

　　线程如果执行run() main()方法结束后，完成逻辑，线程就进入Terminated