多线程: java.lang.Thread
随笔记录，看到哪里记哪里
/**
* Returns a reference to the currently executing thread object.
*
* @return the currently executing thread.
*/
public static native Thread currentThread();博文1 是针对Thread.currentThread()的实践，有点不太好懂
博文2 有详细的解释，但不是太深刻
20190614| 补充：Thread.currentThread() 获取的对象就是当前正在执行的线程，可以是main主线程，也可以是正在运行的新建的线程A，在run方法中Thread.currentThread() 等同于this
/**
* A hint to the scheduler that the current thread is willing to yield
* its current use of a processor. The scheduler is free to ignore this
* hint.
*
* Yield is a heuristic attempt to improve relative progression
* between threads that would otherwise over-utilise a CPU. Its use
* should be combined with detailed profiling and benchmarking to
* ensure that it actually has the desired effect.
*
*
It is rarely appropriate to use this method. It may be useful
* for debugging or testing purposes, where it may help to reproduce
* bugs due to race conditions. It may also be useful when designing
* concurrency control constructs such as the ones in the
* {@link java.util.concurrent.locks} package.
*/
public static native void yield();
博文1
/**
* Causes the currently executing thread to sleep (temporarily cease
* execution) for the specified number of milliseconds, subject to
* the precision and accuracy of system timers and schedulers. The thread
* does not lose ownership of any monitors.
*
* @param millis
* the length of time to sleep in milliseconds
*
* @throws IllegalArgumentException
* if the value of {@code millis} is negative
*
* @throws InterruptedException
* if any thread has interrupted the current thread. The
* interrupted status of the current thread is
* cleared when this exception is thrown.
*/
public static native void sleep(long millis) throws InterruptedException;博文1 一篇很有趣味的文章
博文2 讲的深刻清楚
/**
* Causes this thread to begin execution; the Java Virtual Machine
* calls the run method of this thread.
*
* The result is that two threads are running concurrently: the
* current thread (which returns from the call to the
* start method) and the other thread (which executes its
* run method).
*
* It is never legal to start a thread more than once.
* In particular, a thread may not be restarted once it has completed
* execution.
*
* @exception IllegalThreadStateException if the thread was already
* started.
* @see #run()
* @see #stop()
*/
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 */
}
}
}
private native void start0();
博文1 博文2
源码不太难，自己看也一样可以理解
光看代码觉得差点什么，下面通过实践来探寻原理
// 跳过如下方法
Thread.start()
Thread.run()- 构造方法
public Thread() {
init(null, null, "Thread-" + nextThreadNum(), 0);
}
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
Thread(Runnable target, AccessControlContext acc) {
init(null, target, "Thread-" + nextThreadNum(), 0, acc, false);
}
public Thread(ThreadGroup group, Runnable target) {
init(group, target, "Thread-" + nextThreadNum(), 0);
}
public Thread(String name) {
init(null, null, name, 0);
}
public Thread(ThreadGroup group, String name) {
init(group, null, name, 0);
}
public Thread(Runnable target, String name) {
init(null, target, name, 0);
}
public Thread(ThreadGroup group, Runnable target, String name) {
init(group, target, name, 0);
}
public Thread(ThreadGroup group, Runnable target, String name,
long stackSize) {
init(group, target, name, stackSize);
}看到如上构造器的第一感觉是什么？楼主的第一想法是为什么不用bean优化下，两个构造器能解决的事搞那么多让看那么多代码，但后来仔细想想这个想法是错误的，按楼主想的那样在基础上增加了阅读难度，增加了耦合度
- init方法
整个Thread中有两个init方法，而最终都是指向同一个init方法来处理初始化
/**
* Initializes a Thread with the current AccessControlContext.
* @see #init(ThreadGroup,Runnable,String,long,AccessControlContext,boolean)
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
init(g, target, name, stackSize, null, true);
}/**
* 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(); // 从资源中获取一个线程
SecurityManager security = System.getSecurityManager(); // 防止未知恶意java代码对象
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(); // 检测线程的访问权限，有异常情况报错
/*
* 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();
}在线程构造方法中调用init方法始终 围绕 初始化所属线程组、名字、 Runnable、栈大小等信息；整个过程相当于配置了一个线程工厂，此时只是初始化了所有的配置，线程还没有真正创建，当然资源同样也还没有分配，只有在调用 start() 的时候线程才会真正创建；
- exit方法
private void exit() {
if (group != null) {
group.threadTerminated(this);
group = null;
}
/* Aggressively null out all reference fields: see bug 4006245 */
target = null;
/* Speed the release of some of these resources */
threadLocals = null;
inheritableThreadLocals = null;
inheritedAccessControlContext = null;
blocker = null;
uncaughtExceptionHandler = null;
}
void threadTerminated(Thread t) {
synchronized (this) {
remove(t);
if (nthreads == 0) {
notifyAll();
}
if (daemon && (nthreads == 0) &&
(nUnstartedThreads == 0) && (ngroups == 0))
{
destroy();
}
}
}
private void remove(Thread t) {
synchronized (this) {
if (destroyed) {
return;
}
for (int i = 0 ; i < nthreads ; i++) {
if (threads[i] == t) {
System.arraycopy(threads, i + 1, threads, i, --nthreads - i);
// Zap dangling reference to the dead thread so that
// the garbage collector will collect it.
threads[nthreads] = null;
break;
}
}
}
}exit 方法则是由系统调用，在 Thread 销毁前释放资源；
发现一篇针对Thread讲的很详细的博文，后面的内容暂时就不讲解了，等下次回顾探索一些细节时候再更新。