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爲什麼要有多線程?
從最開始的真空管穿孔打卡，到後來的晶體管批處理系統，再到後來的集成電路多核並行執行。從硬件資源的級別程度逐漸提高，但是軟件如果對硬件資源利用率低，也是一種資源的損耗，所以隨之而來的進程和線程應運而生。
爲什麼有了進程還需要出現線程？
先說一下進程的弊端。
- 相比進程，對於CPU時間片切換，線程是輕量級的。
- 僅有單線程無線程的話，如果單個IO阻塞了整個進程，那麼程序就會Hang住，從而降低了資源利用率。
而線程是現在cpu執行的最小單位，衆所周知，CPU是通過時間片切換來進行類似於併發的效果的。線程的上下文切換的量級遠小於線程，從而可以更大程度的利用CPU資源。
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JAVA的線程狀態
其實學習線程狀態最好是看看Thread類源碼，裏面有一個State的Enum類，寫幾個demo看看~現在截圖一下看看Thread源碼關於線程狀態的註釋：
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;
}
一共6種狀態，具體的轉換圖如下：
查看線程狀態可以使用JDK提供的工具來進行輔助~包括JPS,JSTACK。
JPS獲取java進程號。
JSTACK可以獲取線程堆棧的具體信息
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線程的啓動和銷燬
線程的啓動，不考慮Callable和Future的前提下，只考慮Thread和Runnable。我們看Thread的源碼，我們都是通過調用Start方法而非run方法來啓動線程的。
start方法查看進去，會調用一個start0的native方法。該方法我們就看不到了，因爲是cpp源碼，我們可以找到http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/00cd9dc3c2b5/src/share/native/java/lang/Thread.c 來看具體的Thread的源碼~然後一步一步對應，通過start0-->jvm.cpp裏面的JVM_Start_Thread-->thread.cpp裏面的Thread.Java_thread和Start(需要下載hotspot源碼)-->Thread.start裏面會調用run方法
線程的銷燬
Thread提供了stop和suspend方法，但是都已經過時，其實想想也很正確，A線程調用B線程的stop方法，本身就是一個很奇怪的事情。Thread提供了一個interrupt函數來優雅的中斷一個線程。看源碼又會走到c++源碼中，具體細節不多講了，結果能就是有一個volatile的interrupted的變量變成true，然後呢告知線程，有人向中斷你，僅此而已，至於會不會被中斷，是線程自己的事情。簡單來說就是：
A想中斷B線程，就調用B線程interrupt方法，然後B線程的interrupted boolean變量設置爲true，B線程收到中斷信號，如果不想處理，直接不理這個信號就好，然後調用Thread.interrupted方法來對interrupted變量進行復位。
Thread提供一種IsInterrupted方法來判斷線程是否被中斷。