关于Java线程，你必须掌握的知识点

川长思鸟来 2023-07-10 08:27 9阅读 0赞

## Java线程 ##

> 我为什么要写这篇文章？主要有一下几点：

*  加深自己对Java线程的理解
 *  勘误网上有些错误的知识和认知
 *  通过写文章，来交流思想

> 网上关于Java线程的文章数不胜数，实在太多了，我又怎么写一篇让其他人一看就懂，一看就会，一看就忘不掉的文章呢？

*  用假如我是面试官的场景和面试官的角度去深入分析Java线程相关的知识，下面就开始吧！

### NO1.你熟练使用Java线程，那么Java线程有几种状态？ ###

> 这个时候，大部分同学都应该能回答上来，但是，答案有可能不一样，因为百度上面有五种、六种的很多说法，你说哪个是对的？如下图：  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70]  
> 这个时候你慌不慌，百度是不可靠的，那么可靠的答案在哪里？在**源码**里面

#### Java中Thread的状态（State） ####

/** * A thread state. A thread can be in one of the following states: * <ul> * <li>{@link #NEW} * A thread that has not yet started is in this state. * </li> * <li>{@link #RUNNABLE} * A thread executing in the Java virtual machine is in this state. * </li> * <li>{@link #BLOCKED} * A thread that is blocked waiting for a monitor lock * is in this state. * </li> * <li>{@link #WAITING} * A thread that is waiting indefinitely for another thread to * perform a particular action is in this state. * </li> * <li>{@link #TIMED_WAITING} * A thread that is waiting for another thread to perform an action * for up to a specified waiting time is in this state. * </li> * <li>{@link #TERMINATED} * A thread that has exited is in this state. * </li> * </ul> * * * A thread can be in only one state at a given point in time. * These states are virtual machine states which do not reflect * any operating system thread states. * * @since 1.5 * @see #getState */
 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> * * 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;
 }

> java中线程的状态有6中，其中包括：NEW（新建），RUNNABLE（运行，可能抢到了CPU，也可能没有抢到），BLOCKED（阻塞），WAITING（等待），TIMED\_WAITING（加上时间的等待），TERMINATED（销毁）。

### NO2.线程的状态流转，你能讲一下？ ###

> 我相信很多人都能回答上来，但是，都不是实际的使用经验，而是背下来的。那么，我们来深入的实践一下！

#### 线程状态流转图 ####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 1]

#### sleep时候，线程是什么状态？ ####

> 线程在sleep的时候，是TIMED\_WAITING状态或者RUNNABLE状态，决定因素是sleep的时间是0，或者大于0的值：  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 2]

#### Join的时候，线程是什么状态？ ####

> 线程在join的时候，是TIMED\_WAITING或者WAITING，决定因素是有没有加上时间。  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 3]

#### Wait时候是什么状态？ ####

> 线程在wait的时候，是TIMED\_WAITING或者WAITING，决定因素是有没有加上时间。  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 4]

#### 抢不到锁的时候，是什么状态？ ####

> 抢不到锁的时候是BLOCKED状态  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 5]

### NO3.线程通信 ###

#### 控制两个线程交换打印（手写） ####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 6]

#### 把一副扑克分给4个人（手写） ####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2x2aG9uZ2xlaTE5ODc_size_16_color_FFFFFF_t_70 7]

### NO4.线程安全与可见性 ###

> 安全和可见性，就不展开写了。关注synchronized、volatile，这里面就设计到JMM的知识了。

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