執行緒，有時稱為輕量級進程(Lightweight Process，LWP），是程式執行流程的最小單元。 一個標準的執行緒由執行緒ID，目前指令指標(PC），暫存器集合和堆疊組成。 如果沒有明確的協同機制，則執行緒將彼此獨立執行。每一個程式都至少有一個線程，若程式只有一個線程，那就是程式本身。
線程是進程中的一個實體，是被系統獨立調度和分派的基本單位，線程自己不擁有系統資源，只擁有一點兒在運作中必不可少的資源，但它可與同屬一個進程的其它線程共享進程所擁有的全部資源（共享進程的內存地址空間），因此這些線程都能訪問相同的變量並在同一個堆上分配對象，這就需要實現一種比在進程間共享資料粒度更細的資料共享機制。如果沒有這種同步機制，在多執行緒的情況下會出現無法預料的後果。
一個執行緒可以建立並撤銷另一個執行緒，同一行程中的多個執行緒之間可以並發執行。由於執行緒之間的相互制約，致使執行緒在運行中呈現出間斷性。線程也有就緒、阻斷和運行三種基本狀態。就緒狀態是指執行緒具備運作的所有條件，邏輯上可以運行，在等待處理機；運作狀態是指執行緒佔有處理機正在運作；阻塞狀態是指執行緒在等待一個事件（如某個訊號量），邏輯上不可執行。
執行緒是程式中單一的順序控制流程。進程內一個相對獨立的、可調度的執行單元，是系統獨立調度和分派CPU的基本單位指運作中的程式的調度單位。在單一程式中同時執行多個執行緒完成不同的工作，稱為多執行緒。
一個Java程式的入口是main方法，透過呼叫main方法開始執行，然後依照程式碼邏輯執行，看似沒有其他執行緒參與，其實java程式天生就是多執行緒程序，執行一個main方法其實就是一個名為mian的執行緒和其他執行緒分別執行，參考程式碼如下：
package com.sunld;import java.lang.management.ManagementFactory;import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;/**
* @Title: TestMainThread.java
* @Package com.sunld
* <p>Description:</p>
* @author sunld
* @version V1.0.0
* <p>CreateDate:2017年9月28日 下午3:54:19</p>
*/public class TestMainThread {
public static void main(String[] args) { // 获取Java线程管理MXBean
ThreadMXBean threadMXBean = ManagementFactory.getThreadMXBean(); // 不需要获取同步的monitor和synchronizer信息，仅获取线程和线程堆栈信息
ThreadInfo[] threadInfos = threadMXBean.dumpAllThreads(false, false); // 遍历线程信息，仅打印线程ID和线程名称信息
for (ThreadInfo threadInfo : threadInfos) {
System.out.println("[" + threadInfo.getThreadId() + "] " +
threadInfo.getThreadName());
}
}
}[5] Attach Listener//附加监听 [4] Signal Dispatcher//分发处理发送给JVM信号的线程 [3] Finalizer//调用对象finalize方法的线程 [2] Reference Handler清除Reference的线程 [1] mainmain线程，用户程序入口
/**
* A thread state. A thread can be in one of the following states:
* <ul>
* <li>{@link #NEW}<br>
* A thread that has not yet started is in this state.
* </li>
* <li>{@link #RUNNABLE}<br>
* A thread executing in the Java virtual machine is in this state.
* </li>
* <li>{@link #BLOCKED}<br>
* A thread that is blocked waiting for a monitor lock
* is in this state.
* </li>
* <li>{@link #WAITING}<br>
* A thread that is waiting indefinitely for another thread to
* perform a particular action is in this state.
* </li>
* <li>{@link #TIMED_WAITING}<br>
* 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}<br>
* A thread that has exited is in this state.
* </li>
* </ul>
*
* <p>
* 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>
*
* <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;
}
運行狀態轉換為就緒狀態：當此執行緒主動呼叫yield()方法或在執行過程中失去處理器資源。
運行狀態轉換為死亡狀態：當此執行緒執行體執行完畢或發生了異常。
此處需要特別注意的是：當呼叫執行緒的yield()方法時，執行緒從運行狀態轉換為就緒狀態，但接下來CPU調度就緒狀態中的哪個執行緒具有一定的隨機性，因此，可能會出現A執行緒呼叫了yield()方法後，接下來CPU仍然調度了A執行緒的狀況。
不在執行run方法）在後續的文章中會介紹如何安全的終止一個線程。 。
狀態分析–jvisualvm程式碼-參考（java並發程式設計藝術）package com.sunld;
import java.util.concurrent.TimeUnit;/**
* @Title: TestThreadState.java
* @Package com.sunld
* <p>Description:</p>
* @author sunld
* @version V1.0.0
* <p>CreateDate:2017年9月28日 下午5:14:27</p>
*/public class TestThreadState {
public static void main(String[] args) {
new Thread(new TimeWaiting (), "TimeWaitingThread").start();
new Thread(new Waiting(), "WaitingThread").start(); // 使用两个Blocked线程，一个获取锁成功，另一个被阻塞
new Thread(new Blocked(), "BlockedThread-1").start();
new Thread(new Blocked(), "BlockedThread-2").start();
} //该线程不断地进行睡眠
static class TimeWaiting implements Runnable{
@Override
public void run() {
SleepUtils.second(100);
}
} //该线程在Waiting.class实例上等待
static class Waiting implements Runnable{
@Override
public void run() {
while (true) {
synchronized (Waiting.class) {
try {
Waiting.class.wait();
}catch(InterruptedException e) {
e.printStackTrace();
}
}
}
}
} //该线程在Blocked.class实例上加锁后，不会释放该锁
static class Blocked implements Runnable {
@Override public void run() {
synchronized (Blocked.class) {
while (true) {
SleepUtils.second(100);
}
}
}
}
}class SleepUtils{
public static final void second(long seconds) {
try {
TimeUnit.SECONDS.sleep(seconds);
}catch(InterruptedException e) {
e.printStackTrace();
}
}
}2017-09-28 17:26:47Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.112-b15 mixed mode): //BlockedThread-2线程获取到了Blocked.class的锁"BlockedThread-2" #13 prio=5 os_prio=0 tid=0x000000001f268000 nid=0x3754 waiting on condition [0x000000002009f000] java.lang.Thread.State: TIMED_WAITING (sleeping) //BlockedThread-1线程阻塞在获取Blocked.class示例的锁上"BlockedThread-1" #12 prio=5 os_prio=0 tid=0x000000001f266800 nid=0x89c waiting for monitor entry [0x000000001ff9f000] java.lang.Thread.State: BLOCKED (on object monitor) //WaitingThread线程在Waiting实例上等待"WaitingThread" #11 prio=5 os_prio=0 tid=0x000000001f260800 nid=0x4d08 in Object.wait() [0x000000001fe9f000] java.lang.Thread.State: WAITING (on object monitor) //TimeWaitingThread线程处于超时等待"TimeWaitingThread" #10 prio=5 os_prio=0 tid=0x000000001f25f000 nid=0x42ac waiting on condition [0x000000001fd9e000] java.lang.Thread.State: TIMED_WAITING (sleeping)
以上是Java並發之線程的使用以及基本概念的詳細內容。更多資訊請關注PHP中文網其他相關文章！
