[java] Thread類詳解
目錄
本篇部落格基於jdk1.8
Thread類概述
Thread的一個例項,即是一個程式中執行的一個執行緒,java虛擬機器允許一個應用程式同時有多個併發的執行緒執行。
每一個執行緒都有一個優先順序。優先順序高的執行緒比優先順序低的執行緒優先執行。每一個執行緒也都可能被標記為一個守護執行緒。當代碼執行到某一個執行緒去建立另一個執行緒時,新建立的執行緒的優先順序會初始化成和建立它的執行緒一樣。並且當且僅當建立執行緒為守護執行緒時,被建立執行緒才會被設為守護執行緒。
當Java虛擬機器啟動時,通常會有一個單獨的非守護程序執行緒(通常這個非守護執行緒會去呼叫一個指定的類的main方法)。Java虛擬機器器繼續執行執行緒,直到以下任一一種情況發生:
Runtime類的exit方法被呼叫並且security manager(安全管理器)允許退出操作操作發生。
所有的非守護執行緒全部都已經終結,要麼通過從呼叫返回到執行方法或由丟擲超出執行的異常方法。
建立一個新的執行執行緒有兩種方法:
宣告一個Thread類的子類。這個子類應當去重寫Thread類裡的run方法。然後子類的例項就可以分配和啟動。例如,計算大於指定值的素數的執行緒可以寫成如下:
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
...
}
}
下面的程式碼就可以建立一個執行緒並啟動它:
PrimeThread p = new PrimeThread(143); p.start();
建立執行緒的另一種方法是宣告一個類實現Runable介面,這個類需要實現run方法,然後這個類的例項就可以在建立執行緒時被當做引數傳遞進去,並且啟動,同一個例子的另一種樣式如下:
class PrimeRun implements Runnable {
long minPrime;
PrimeRun(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
...
}
}
然後,下面的程式碼就可以建立一個執行緒並啟動它:
PrimeRun p = new PrimeRun(143);
new Thread(p).start();
每個執行緒都有用於標識目的的名稱。多個執行緒可能具有相同的名稱。如果建立一個執行緒時未指定名稱,則會為其生成一個新名稱。
除非另有說明,否則,向Thread類中的建構函式或方法傳遞null引數將導致丟擲NullPointerException。
另外一點特別有意思的是,Thread類本身就是實現了Runnable介面的一個類。
成員變數
private volatile String name; //執行緒的名字
private int priority; //執行緒優先順序
/* Whether or not to single_step this thread. */
private boolean single_step;
/* Whether or not the thread is a daemon thread. */
private boolean daemon = false; //是否是守護執行緒
/* What will be run. */
private Runnable target; //將會被執行的Runnable.
/* The group of this thread */
private ThreadGroup group; //這個執行緒的組
/* The context ClassLoader for this thread */
private ClassLoader contextClassLoader; //這個執行緒的上下文
/* The inherited AccessControlContext of this thread */
private AccessControlContext inheritedAccessControlContext; //繼承的請求控制
/* For autonumbering anonymous threads. */
private static int threadInitNumber; //預設執行緒的自動編號
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null; //當前執行緒附屬的ThreadLocal,而ThreadLocalMap會被ThreadLocal維護)
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
// 主要作用:為子執行緒提供從父執行緒那裡繼承的值
//在建立子執行緒時,子執行緒會接收所有可繼承的執行緒區域性變數的初始值,以獲得父執行緒所具有的值
// 建立一個執行緒時如果儲存了所有 InheritableThreadLocal 物件的值,那麼這些值也將自動傳遞給子執行緒
//如果一個子執行緒呼叫 InheritableThreadLocal 的 get() ,那麼它將與它的父執行緒看到同一個物件
/*
* The requested stack size for this thread, or 0 if the creator did
* not specify a stack size. It is up to the VM to do whatever it
* likes with this number; some VMs will ignore it.
*/
private long stackSize; //該執行緒請求的堆疊大小 預設一般都是忽略
/*
* Thread ID
*/
private long tid; // 每個執行緒都有專屬ID,但名字可能重複
/* For generating thread ID */
private static long threadSeqNumber; //用來生成thread ID
/* Java thread status for tools,
* initialized to indicate thread 'not yet started'
*/
private volatile int threadStatus = 0; //標識執行緒狀態,預設是執行緒未啟動
/* The object in which this thread is blocked in an interruptible I/O
* operation, if any. The blocker's interrupt method should be invoked
* after setting this thread's interrupt status.
*/
private volatile Interruptible blocker; //阻塞器鎖,主要用於處理阻塞情況
/**
* The minimum priority that a thread can have.
*/
public final static int MIN_PRIORITY = 1;//執行緒的優先順序中最小的
/**
* The default priority that is assigned to a thread.
*/
public final static int NORM_PRIORITY = 5; //執行緒的優先順序中第二的同時也是預設的優先順序
/**
* The maximum priority that a thread can have.
*/
public final static int MAX_PRIORITY = 10; //最高的優先順序
構造方法
據完全統計,Thread類裡一共提供了九個構造方法...其中有一個是非public的方法。
但是這九個構造方法事實上最後都是呼叫了一個叫做init的方法,該方法定義如下:
/**
* Initializes a Thread.
*
* @param g the Thread group
* @param target the object whose run() method gets called
* @param name the name of the new Thread
* @param stackSize the desired stack size for the new thread, or
* zero to indicate that this parameter is to be ignored.
* @param acc the AccessControlContext to inherit, or
* AccessController.getContext() if null
* @param inheritThreadLocals if {@code true}, inherit initial values for
* inheritable thread-locals from the constructing thread
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null) {
g = security.getThreadGroup();
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup();
}
}
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
該方法一共六個引數:
1.ThreadGroup 執行緒組
2.Runnable target 一個Runnable物件,即呼叫run方法的物件
3.String name 新執行緒的名字
4.long stackSize 新執行緒分配所需堆疊的大小
5.AccessControlContext acc 許可權控制
6.boolean inheritThreadLocals 是否繼承ThreadLocals, 這個引數總是true。
方法的實現大致總結起來就是,給變數賦初值,大部分的初值都是通過該執行緒的父執行緒賦值,獲取父執行緒物件的程式碼為:
Thread parent = currentThread();
而currentThread方法的實現是一個本地方法。
start方法
start方法的作用就是讓該執行緒開始執行,java虛擬機器會呼叫這個執行緒的run方法。
執行start方法的結果是兩個執行緒會併發的執行,當前執行緒即執行start方法的執行緒,以及另一個呼叫run方法的執行緒。
啟動一個執行緒兩次永遠是非法的,會丟擲IllegalThreadStateException。程式碼如下:
public synchronized void start() {
/**
* This method is not invoked for the main method thread or "system"
* group threads created/set up by the VM. Any new functionality added
* to this method in the future may have to also be added to the VM.
*
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented. */
group.add(this);
boolean started = false;
try {
start0();
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
可以看到上來會驗證執行緒的狀態,如果執行緒已啟動則拋異常。核心實現程式碼為方法start0,是一個本地方法,在這個本地方法的實現當中,虛擬機器會去呼叫該執行緒的run方法,在java程式碼中並看不出這一點。
這就是我們可以通過start方法啟動執行緒,並且run方法會執行的原因所在。
run方法
Thread類實現了Runable介面,自然也就實現了run方法,程式碼如下
@Override
public void run() {
if (target != null) {
target.run();
}
}
target定義如下:
/* What will be run. */
private Runnable target;
target就是Runnable的一個例項,是Thread類中執行run的物件。注意:這裡的程式碼同時說明了我們new一個Thread的時候問什麼要傳入一個Runable的物件,因為事實上後來這個Thread啟動呼叫run方法的時候,就是呼叫的這個Runable的run方法。
問題:Thread直接呼叫run()和start()方法的區別?
run()方法: 在本執行緒內呼叫該Runnable物件的run()方法,可以重複多次呼叫;
start()方法:啟動一個執行緒,然後Java虛擬機器會呼叫該Runnable物件,即new一個Thread的時候傳入的Runable物件,的run()方法,不能多次啟動一個執行緒。
你呼叫run()方法的時候,只會是在原來的執行緒中呼叫,沒有新的執行緒啟動,start()方法才會啟動新執行緒。
sleep方法
有兩個過載的sleep方法。其中一個程式碼如下:
public static void sleep(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}
sleep(millis);
}
在呼叫另一個sleep方法之前,就是做了一些range check,即範圍檢查,然後就直接呼叫了另一個native的sleep方法。
yield方法
程式碼如下
public static native void yield();
作用是使當前執行緒從執行狀態(執行狀態)變為可執行態(就緒狀態)。也是一個native方法。
interrupt方法
public void interrupt() {
if (this != Thread.currentThread())
checkAccess();
synchronized (blockerLock) {
Interruptible b = blocker;
if (b != null) {
interrupt0(); // Just to set the interrupt flag
b.interrupt(this);
return;
}
}
interrupt0();
}
實現也是呼叫了一個interrupt0的本地方法,有一句關鍵的註釋:只是去set了interrupt flag,即不會真正的去停止執行緒,關於這一部分的內容,可以參考我的另一篇部落格:停止執行緒。
join方法
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
join的作用就是使得當前執行緒阻塞,一直到呼叫join方法的執行緒物件執行完run方法中的任務且銷燬之後才繼續執行後面的程式碼。
從上面的程式碼實現來看,join方法是通過和wait方法配合實現的。
getState方法
該方法是返回執行緒的狀態,它的實現很有意思,如下:
public State getState() {
// get current thread state
return sun.misc.VM.toThreadState(threadStatus);
}
public static State toThreadState(int var0) {
if ((var0 & 4) != 0) {
return State.RUNNABLE;
} else if ((var0 & 1024) != 0) {
return State.BLOCKED;
} else if ((var0 & 16) != 0) {
return State.WAITING;
} else if ((var0 & 32) != 0) {
return State.TIMED_WAITING;
} else if ((var0 & 2) != 0) {
return State.TERMINATED;
} else {
return (var0 & 1) == 0 ? State.NEW : State.RUNNABLE;
}
}
toThreadState方法的實現就是根據執行緒狀態的數字,返回對應的狀態而已,它們只是有一個mapping的關係。State的定義則是一個列舉,如下:
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;
}
可以看到java的執行緒一共是有六種狀態的。
holdsLock方法判斷執行緒是否持有鎖物件
/**
* Returns <tt>true</tt> if and only if the current thread holds the
* monitor lock on the specified object.
*
* <p>This method is designed to allow a program to assert that
* the current thread already holds a specified lock:
* <pre>
* assert Thread.holdsLock(obj);
* </pre>
*
* @param obj the object on which to test lock ownership
* @throws NullPointerException if obj is <tt>null</tt>
* @return <tt>true</tt> if the current thread holds the monitor lock on
* the specified object.
* @since 1.4
*/
public static native boolean holdsLock(Object obj);
為什麼wait和notify方法不在Thread類裡定義
需要說明為什麼把這些方法放在Object類裡是有意義的,還有不把它放在Thread類裡的原因。一個很明顯的原因是JAVA提供的鎖是物件級的而不是執行緒級的,每個物件都有鎖,通過執行緒獲得。如果執行緒需要等待某些鎖那麼呼叫物件中的wait()方法就有意義了。如果wait()方法定義在Thread類中,執行緒正在等待的是哪個鎖就不明顯了。簡單的說,由於wait,notify和notifyAll都是鎖級別的操作,所以把他們定義在Object類中因為鎖屬於物件
總結
經過上面的分析,可以發現事實上Thread類因為設計到執行緒的操作,因此大量的還是依靠了native方法實現。java核心邏輯並不多。