学习一下线程相关东西
什么是线程?什么是进程?
进程:（Process）是计算机中的程序关于某数据集合上的一次运行活动，是系统进行资源分配的基本单位，是操作系统结构的基础.Android 系统中 一个App就是一个进程(或多个进程)
线程:（英语：thread）是操作系统能够进行运算调度的最小单位。它被包含在进程之中，是进程中的实际运作单位。Android中 App运行在主线程中
1:创建线程方式
方式1.创建Runable 方式
Runnable runnable= new Runnable(){
@Override
public void run() {
}
};
new Thread(runnable).start();
2.方式2 实现Runable 接口方式
new Thread(){
@Override
public void run() {
super.run();
}
}.start();
2.Thread.run() 和Thread.start()区别
run:是Runable接口的方法, Thread实现了这个接口,Thread调用run方式只是调用了Thread 的run方法而已
/**
* If this thread was constructed using a separate
* {@code Runnable} run object, then that
* {@code Runnable} object's {@code run} method is called;
* otherwise, this method does nothing and returns.
* <p>
* Subclasses of {@code Thread} should override this method.
*
* @see #start()
* @see #stop()
* @see #Thread(ThreadGroup, Runnable, String)
*/
@Override
public void run() {
if (target != null) {
target.run();
}
}
start: 使此线程开始执行 调用Native方法创建线程 并且执行线程线程变成 started状态
/**
* Causes this thread to begin execution; the Java Virtual Machine
* calls the {@code run} method of this thread.
* <p>
* The result is that two threads are running concurrently: the
* current thread (which returns from the call to the
* {@code start} method) and the other thread (which executes its
* {@code run} method).
* <p>
* It is never legal to start a thread more than once.
一个线程启动多次是不合法的。
* In particular, a thread may not be restarted once it has completed
* execution.
特别是，线程一旦完成就可能无法重新启动执行
*
* @throws 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".
*/
// Android-changed: Replace unused threadStatus field with started field.
// The threadStatus field is unused on Android.
// if (threadStatus != 0)
//多次执行后会抛出异常,除非执行结束后挂起
if (started)
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);
// Android-changed: Use field instead of local variable.
// It is necessary to remember the state of this across calls to this method so that it
// can throw an IllegalThreadStateException if this method is called on an already
// started thread.
// boolean started = false;
started = false;
try {
// Android-changed: Use Android specific nativeCreate() method to create/start thread.
// start0();
nativeCreate(this, stackSize, daemon);
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 */
}
}
}
4.线程状态
Thread.State 线程状态
-
Thread.State.NEW Thread thread=new Thread;
线程刚创建,尚未启动的线程的线程状态 -
Thread.State.RUNNABLE thread.start();
可运行线程的线程状态。处于可运行状态的线程正在Java虚拟机中执行，但它可能正在等待来自操作系统的其他资源,例如处理器。 -
Thread.State.BLOCKED 该状态是线程因为某种原因放弃了cpu使用权限，暂时停止运行
被阻止等待监视器锁定的线程的线程状态。处于阻塞状态的线程正在等待监视器锁进入同步的块/方法，或者在调用Object.wait后重新进入同步块/方法。
阻塞的情况有三种：
1）等待：调用了wait(),线程进入等待阻塞状态。此时线程需要等待某项工作的完成
2）同步：当线程获取synchronized同步锁失败（此时锁被其他线程暂时占用），线程进入同步阻塞状态。此时该线程需要等其他线程使用完锁。
3）其他：当调用了sleel()或join()或发出了I/O请求时，线程进入阻塞状态。此时线程需要等sleep()时间结束或被打断、join()中断、I/O处理完成，线程才能重新进入就绪状态。
- Thread.State.WAITING
等待线程的线程状态。由于调用以下方法之一，线程处于等待状态：- Object.wait :无超时的对象等待
- Thread.join无超时的线程联接
- 锁定Support.park
处于等待状态的线程正在等待另一个线程执行特定操作。例如，对某个对象调用了Object.wait（）的线程正在等待另一个线程对该对象调用Object.notify（）或Object.notifyAll（）。一个调用了thread.join（）的线程正在等待指定的线程终止。
- Thread.State.TIMED_WAITING
处于等待状态的线程正在等待另一个线程执行特定操作。例如，对某个对象调用了Object.wait（）的线程正在等待另一个线程对该对象调用Object.notify（）或Object.notifyAll（）。一个调用了thread.join（）的线程正在等待指定的线程终止。 - Thread.State.TERMINATED
具有指定等待时间的等待线程的线程状态。由于在指定的正等待时间内调用以下方法之一，线程处于定时等待状态：- Thread.sleep
- Object.wait with timeout
- Thread.join with timeout
- LockSupport.parkNanos
- LockSupport.parkUntil
1)线程中run()或call()执行完成，正常结束
2)线程抛出Exception或error，异常退出
3)线程调用stop()结束—容易导致死锁，不建议
注意:
start()一个线程只能执行一次,再次执行会报异常
started赋值为true再次运行直接抛异常
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".
*/
// Android-changed: Replace unused threadStatus field with started field.
// The threadStatus field is unused on Android.
// if (threadStatus != 0)
if (started)
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);
// Android-changed: Use field instead of local variable.
// It is necessary to remember the state of this across calls to this method so that it
// can throw an IllegalThreadStateException if this method is called on an already
// started thread.
// boolean started = false;
started = false;
try {
// Android-changed: Use Android specific nativeCreate() method to create/start thread.
// start0();
nativeCreate(this, stackSize, daemon);
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 */
}
}
}
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