Thread類的實例方法
start()
start方法內部會調用方法start方法啟動一個線程，該線程返回start方法，同時Java虛擬機調用native start0啟動另一個線程調用run方法，此時有兩個線程并行執行；
我們來分析下start0方法，start0到底是如何調用run方法的
Thread類里有一個本地方法叫registerNatives，此方法註冊一些本地方法給Thread類使用
在OpenJDK官網找到Thread.c
#include "jni.h"
#include "jvm.h"
#include "java_lang_Thread.h"
#define THD "Ljava/lang/Thread;"
#define OBJ "Ljava/lang/Object;"
#define STE "Ljava/lang/StackTraceElement;"
#define ARRAY_LENGTH(a) (sizeof(a)/sizeof(a[0]))
static JNINativeMethod methods[] = {
{"start0", "()V", (void *)&JVM_StartThread}, //Java中Thread類的start方法所調用的start0方法
{"stop0", "(" OBJ ")V", (void *)&JVM_StopThread},
{"isAlive", "()Z", (void *)&JVM_IsThreadAlive},
{"suspend0", "()V", (void *)&JVM_SuspendThread},
{"resume0", "()V", (void *)&JVM_ResumeThread},
{"setPriority0", "(I)V", (void *)&JVM_SetThreadPriority},
{"yield", "()V", (void *)&JVM_Yield},
{"sleep", "(J)V", (void *)&JVM_Sleep},
{"currentThread", "()" THD, (void *)&JVM_CurrentThread},
{"countStackFrames", "()I", (void *)&JVM_CountStackFrames},
{"interrupt0", "()V", (void *)&JVM_Interrupt},
{"isInterrupted", "(Z)Z", (void *)&JVM_IsInterrupted},
{"holdsLock", "(" OBJ ")Z", (void *)&JVM_HoldsLock},
{"getThreads", "()[" THD, (void *)&JVM_GetAllThreads},
{"dumpThreads", "([" THD ")[[" STE, (void *)&JVM_DumpThreads},
};
......根據關鍵字”JVM_StartThread”再找到jvm.cpp
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
JVMWrapper("JVM_StartThread");
JavaThread *native_thread = NULL;
bool throw_illegal_thread_state = false;
{
MutexLocker mu(Threads_lock);
if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {
throw_illegal_thread_state = true;
} else {
jlong size =
java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread));
size_t sz = size > 0 ? (size_t) size : 0;
native_thread = new JavaThread(&thread_entry, sz); //請看這裏，實例化了一個線程native_thread
if (native_thread->osthread() != NULL) {
// Note: the current thread is not being used within "prepare".
native_thread->prepare(jthread);
}
}
}sz是大小參數，忽略之，我們看thread_entry是什麼
static void thread_entry(JavaThread* thread, TRAPS) {
HandleMark hm(THREAD);
Handle obj(THREAD, thread->threadObj());
JavaValue result(T_VOID);
JavaCalls::call_virtual(&result,
obj,
KlassHandle(THREAD, SystemDictionary::Thread_klass()),
vmSymbols::run_method_name(), //請看這裏，jvm調用run_method_name方法
vmSymbols::void_method_signature(),
THREAD);
}run_method_name在vmSymbols.hpp被定義
/* common method and field names */
template(run_method_name, "run") //run_method_name的名稱是"run"簡言之：當前線程調用start方法通知ThreadGroup當前線程可以運行了，可以被加入了，當前線程啟動后，當前線程狀態為”Runnable”。另一個線程等待CPU時間片，調用run方法（線程真正執行）。產生一個異步執行的效果；
用start方法來啟動線程，真正實現了多線程運行，這時無需等待run方法體代碼執行完畢而直接繼續執行下面的代碼。
代碼如下
public class MyThread03 extends Thread{
public void run()
{
try
{
for (int i = 0; i < 3; i++)
{
Thread.sleep((int)(Math.random() * 1000));
System.out.println("run = " + Thread.currentThread().getName());
}
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
public static void main(String[] args)
{
MyThread03 mt = new MyThread03();
mt.start();
try
{
for (int i = 0; i < 3; i++)
{
Thread.sleep((int)(Math.random() * 1000));
System.out.println("run = " + Thread.currentThread().getName());
}
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}執行結果如下,可以看到，Thead-0和main線程交叉執行，是無序的。很好理解，因為main和Thread-0在爭搶CPU資源，這個過程是無序的。
run = main
run = Thread-0
run = main
run = main
run = Thread-0
run = Thread-0再看一個例子，代碼如下
public class MyThread04 extends Thread{
public void run()
{
System.out.println(Thread.currentThread().getName());
}
public static void main(String[] args)
{
MyThread04 mt0 = new MyThread04();
MyThread04 mt1 = new MyThread04();
MyThread04 mt2 = new MyThread04();
mt0.start();
mt1.start();
mt2.start();
}
}執行結果如下
Thread-0
Thread-2
Thread-1我們依次啟動mt0,mt1,mt2，這說明線程啟動順序也是無序的。因為start方法僅僅返回調用，線程想要執行必須得到CPU時間片再執行run方法，CPU時間片的獲得是無序的。
run()
run方法是Thread類的一個普通方法，執行run方法其實是單線程執行
public class MyThread05 extends Thread{
public void run()
{
System.out.println("run = " + Thread.currentThread().getName());
}
public static void main(String[] args)
{
MyThread05 mt = new MyThread05();
mt.run();
try
{
for (int i = 0; i < 3; i++)
{
Thread.sleep((int)(Math.random() * 1000));
System.out.println("run = " + Thread.currentThread().getName());
}
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}輸出結果如下
run = main
run = main
run = main
run = mainmain線程循環了3次，run方法1次，結果是main線程執行了四次，我們寫在run方法體內的被main線程執行，這說明調用run方法執行多線程是不可行的。
isAlive()
判斷線程是否存活
public class MyThread06 extends Thread{
public void run()
{
System.out.println("run = " + this.isAlive());
}
public static void main(String[] args) throws Exception
{
MyThread06 mt = new MyThread06();
System.out.println("begin == " + mt.isAlive());
mt.start();
Thread.sleep(100);
System.out.println("end == " + mt.isAlive());
}
}輸出結果如下,增加0.1秒延遲，讓線程執行完
begin == false
run = true
end == false可以看到，執行前false，執行中true，執行后false
getId()
返回線程的標識符，線程ID是正值，線程ID在生命周期內不會變化，當線程終止了，線程ID可能會被重用
getName()
返回線程名稱
getPriority()和setPriority(int)
返回優先級和設置優先級
優先級越高的線程獲取CPU時間片的概率越高
請看如下的例子
public class MyThread07_0 extends Thread{
public void run()
{
System.out.println("MyThread07_0 run priority = " +
this.getPriority());
}
public static void main(String[] args)
{
System.out.println("main thread begin, priority = " +
Thread.currentThread().getPriority());
System.out.println("main thread end, priority = " +
Thread.currentThread().getPriority());
MyThread07_0 thread = new MyThread07_0();
thread.start();
}
}運行結果如下
main thread begin, priority = 5
main thread end, priority = 5
MyThread07_0 run priority = 5 線程的默認優先級是5
再看如下的例子
public class MyThread07_1 extends Thread {
public void run()
{
System.out.println("MyThread07_1 run priority = " +
this.getPriority());
MyThread07_0 thread = new MyThread07_0();
thread.start();
}
public static void main(String[] args)
{
System.out.println("main thread begin, priority = " +
Thread.currentThread().getPriority());
System.out.println("main thread end, priority = " +
Thread.currentThread().getPriority());
MyThread07_1 thread = new MyThread07_1();
thread.start();
}
}我們在MyThread07_1線程內部啟動MyThread07_0線程，我們觀察MyThread07_1和MyThread07_0的優先級有什麼關係。
運行結果如下
main thread begin, priority = 5
main thread end, priority = 5
MyThread07_1 run priority = 5
MyThread07_0 run priority = 5MyThread07_0和MyThread07_1線程的優先級一致，說明線程具有繼承性。
現在我們來設置優先級
public class MyThread08 {
static class MyThread08_0 extends Thread {
public void run() {
long beginTime = System.currentTimeMillis();
for (int j = 0; j < 1000000; j++) {}
long endTime = System.currentTimeMillis();
System.out.println("★★★★ MyThread08_0 use time = " +
(endTime - beginTime));
}
}
static class MyThread08_1 extends Thread {
public void run()
{
long beginTime = System.currentTimeMillis();
for (int j = 0; j < 1000000; j++){}
long endTime = System.currentTimeMillis();
System.out.println("☆☆☆☆ MyThread08_1 use time = " +
(endTime - beginTime));
}
}
public static void main(String[] args)
{
for (int i = 0; i < 5; i++)
{
MyThread08_0 mt0 = new MyThread08_0();
mt0.setPriority(5);
mt0.start();
MyThread08_1 mt1 = new MyThread08_1();
mt1.setPriority(4);
mt1.start();
}
}
}我們給MyThread08_0線程設置更高的優先級5
運行結果如下
★★★★ MyThread08_0 use time = 7
☆☆☆☆ MyThread08_1 use time = 4
★★★★ MyThread08_0 use time = 18
★★★★ MyThread08_0 use time = 16
★★★★ MyThread08_0 use time = 20
★★★★ MyThread08_0 use time = 17
☆☆☆☆ MyThread08_1 use time = 0
☆☆☆☆ MyThread08_1 use time = 10
☆☆☆☆ MyThread08_1 use time = 9
☆☆☆☆ MyThread08_1 use time = 8可以看到MyThread08_0先執行的次數更多，輸出結果為實心五角星的這個。
多運行幾次，都會是MyThread08_0先打印完，每次結果都不盡相同，CPU會盡量先讓MyThread08_0執行完。
isDaemon()和setDaemon(boolean)
isDaemon方法判斷是否是守護線程；
setDaemon設置守護線程
在Java中有兩類線程：User Thread(用戶線程)、Daemon Thread(守護線程)
我們自定義的線程和main線程都是用戶線程，我們熟知的GC(垃圾回收器)就是守護線程。守護線程是用戶線程的“奴僕”，當用戶線程執行完畢，守護線程就會終止，因為它沒有存在的必要了。
如用戶線程執行結束，GC無垃圾可回收，它只能死亡
看如下代碼
public class MyThread09 extends Thread{
private int i = 0;
public void run()
{
try
{
while (true)
{
i++;
System.out.println(Thread.currentThread().getName()+" i = " + i);
Thread.sleep(1000);
}
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
public static void main(String[] args)
{
try
{
MyThread09 mt = new MyThread09();
mt.setDaemon(true);
mt.start();
Thread.sleep(5000);
System.out.println("現在是"+Thread.currentThread().getName()+"線程");
Thread.sleep(1);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}我們自定義MyThread09線程的run方法里是死循環，如果是用戶線程，它應該永遠地執行下去，現在把它設置成守護線程。
注意：mt.setDaemon(true);要在mt.start();之前，見
否則會拋出IllegalThreadStateException異常
運行結果如下
Thread-0 i = 1
Thread-0 i = 2
Thread-0 i = 3
Thread-0 i = 4
Thread-0 i = 5
現在是main線程
Thread-0 i = 6
MyThread09變成了守護線程，它的使命已經完成。現在是main線程Thread.sleep(5000)的目的是使main線程沉睡5s，即用戶線程（main線程）仍在執行，此時main線程輸出，再沉睡1ms，當main線程執行完畢，守護線程就沒有存在的意義了，即死亡；
main線程總共執行了大約5001ms（略大於這個數值），Thread-0打印到i=6，說明守護線程在main線程之後死亡，這個時間差極小
interrupt()
設置中斷標誌位，無法中斷線程
public class MyThread10 extends Thread{
public void run()
{
for (int i = 0; i < 500000; i++)
{
System.out.println("i = " + (i + 1));
}
}
public static void main(String[] args)
{
try
{
MyThread10 mt = new MyThread10();
mt.start();
Thread.sleep(2000);
mt.interrupt();
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}輸出結果如下
......
i = 499993
i = 499994
i = 499995
i = 499996
i = 499997
i = 499998
i = 499999
i = 500000可以看到，interrupt()沒有中斷線程，interrupt()後續將會詳細講解
isInterrupted()
判斷線程是否被中斷
join()
等待這個線程死亡，舉例說明：
線程A執行join方法，會阻塞線程B，線程A join方法執行完畢，才能執行線程B
代碼如下
public class MyThread11 extends Thread{
public void run()
{
try
{
int secondValue = (int)(Math.random() * 1000);
System.out.println(secondValue);
Thread.sleep(secondValue);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
public static void main(String[] args) throws Exception
{
MyThread11 mt = new MyThread11();
mt.start();
mt.join();
System.out.println("MyThread11執行完畢之後我再執行");
}
}輸出結果如下
75
MyThread11執行完畢之後我再執行可以看到，main線程在mt線程之後執行。mt調用join方法，使main線程阻塞，待mt線程執行完畢，方可執行main線程。
Thread類的靜態方法
currentThread()
返回當前正在執行線程的引用
public class MyThread12 extends Thread{
static
{
System.out.println("靜態塊的打印：" +
Thread.currentThread().getName());
}
public MyThread12()
{
System.out.println("構造方法的打印：" +
Thread.currentThread().getName());
}
public void run()
{
System.out.println("run()方法的打印：" +
Thread.currentThread().getName());
}
public static void main(String[] args)
{
MyThread12 mt = new MyThread12();
mt.start();
}
}輸出結果
靜態塊的打印：main
構造方法的打印：main
run()方法的打印：Thread-0可以看到，構造方法和靜態塊是main線程在調用，重寫的run方法是線程自己在調用。
再看個例子
public class MyThread13 extends Thread{
public MyThread13()
{
System.out.println("MyThread13----->Begin");
System.out.println("Thread.currentThread().getName()----->" +
Thread.currentThread().getName());
System.out.println("this.getName()----->" + this.getName());
System.out.println("MyThread13----->end");
}
public void run()
{
System.out.println("run----->Begin");
System.out.println("Thread.currentThread().getName()----->" +
Thread.currentThread().getName());
System.out.println("this.getName()----->" + this.getName());
System.out.println("run----->end");
}
public static void main(String[] args)
{
MyThread13 mt = new MyThread13();
mt.start();
}
}輸出結果
MyThread13----->Begin
Thread.currentThread().getName()----->main
this.getName()----->Thread-0
MyThread13----->end
run----->Begin
Thread.currentThread().getName()----->Thread-0
this.getName()----->Thread-0
run----->end可以看到，執行MyThread13構造方法的線程是main，執行MyThread13的線程是Thread-0（當前線程）,run方法就是被線程實例所執行。
sleep(long)
讓當前線程沉睡若干毫秒
public class MyThread14 extends Thread{
public void run()
{
try
{
System.out.println("run threadName = " +
this.getName() + " begin");
Thread.sleep(2000);
System.out.println("run threadName = " +
this.getName() + " end");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
public static void main(String[] args)
{
MyThread14 mt = new MyThread14();
mt.start();
}
}輸出結果如下
run threadName = Thread-0 begin
run threadName = Thread-0 end打印完第一句兩秒后打印第二句。
yield()
當前線程放棄CPU的使用權，這裏的放棄是指當前線程少用CPU資源，最後線程還是會執行完成
public class MyThread15 extends Thread {
public void run()
{
long beginTime = System.currentTimeMillis();
int count = 0;
for (int i = 0; i < 5000000; i++)
{
Thread.yield();
count = count + i + 1;
}
long endTime = System.currentTimeMillis();
System.out.println("用時：" + (endTime - beginTime) + "毫秒！");
}
public static void main(String[] args)
{
MyThread15 mt = new MyThread15();
mt.start();
}
}輸出結果如下
用時：4210毫秒！可以看到，任務執行完畢，當我們把Thread.yield();註釋掉，執行時間只需要7ms。說明當前線程放棄了一些CPU資源。
interrupted()
判斷當前線程是否中斷，靜態版的isInterrupted方法。多線程中斷機制，後續會詳細解析。
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