線程之我見(一)

概念

1. 是操作系統能夠進行運算調度的最小單位。
2. 是進程中的實際運作單位。
3. 是系統分配處理器時間資源的基本單元，即程序執行的最小單位

順便介紹下進程

1. 進程是在系統中正在運行的一個應用程序；程序一旦運行就是進程；
2. 進程是資源分配的最小單位
3. 進程可以包含多個線程

線程狀態

• java線程狀態定義在java.lang.Thread.State枚舉類中；分別是

1. NEW（初始狀態）
2. RUNNABLE （就緒、執行中狀態）
3. BLOCKED（阻塞狀態）
4. WAITING（等待狀態）
5. TIMED_WAITING （超時等待狀態）
6. TERMINATED（終止狀態）

• 源碼如下：

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;
    }

• 狀態詳解

NEW（初始狀態）

定義

線程剛創建，但是沒有調用start方法時候的狀態

實例

		ThreadDemo threadDemo = new ThreadDemo ();
        System.out.println(threadDemo .getState());//NEW

RUNNABLE （就緒、執行中狀態）

定義

調用start()方法之後.(線程狀態爲 RUNNABLE 就緒、執行中狀態)

1.此狀態下線程可能真的在運行RUNNING(執行中)， 也有可能是在等待CPU資源(READY就緒)
2.RUNNING(執行中)和(READY就緒)只是方便理解添加的兩個狀態，實際上線程對象沒有這兩個狀態

實例

        ThreadDemo threadDemo = new ThreadDemo ();
        threadDemo .start();
        System.out.println(threadDemo .getState());//RUNNABLE 

BLOCKED（阻塞狀態）

定義

等待獲取一個監視器鎖以便進入synchronized塊或者方法時候的狀態。

實例

• 線程類MyThread

package com.example.thread;

import com.example.util.DateUtil;

import java.util.Date;

public class MyThread extends Thread {
    private Object lock;
    public MyThread(Object lock) {
        this.lock = lock;
    }
    @Override
    public void run() {
        synchronized (lock) {
            System.out.println(Thread.currentThread().getName() + " entry synchronized " + DateUtil.format(new Date(), "yyyy-MM-dd HH:mm:ss"));
            try {
                Thread.sleep(10000);//休眠10s
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " done " + DateUtil.format(new Date(), "yyyy-MM-dd HH:mm:ss"));
        }
    }
}

• 測試類

package com.example.springdemo01.thread;

public class ThreadDemo02 {
    public static void main(String[] args) throws InterruptedException {
        Object lock = new Object();
        MyThread thread1 = new MyThread(lock);
        thread1.start();
        MyThread thread2 = new MyThread(lock);
        thread2.start();
        Thread.sleep(1000);//等1s再檢查狀態，以便線程thread2完全啓動成功，進入獲取lock對象的鎖階段
        System.out.println(thread2.getName() + " 線程狀態 " + thread2.getState());
    }
}

• 控制檯打印

Thread-0 entry synchronized 2020-06-14 18:02:12
Thread-1 線程狀態 BLOCKED
Thread-0 done 2020-06-14 18:02:22
Thread-1 entry synchronized 2020-06-14 18:02:22
Thread-1 done 2020-06-14 18:02:32

Process finished with exit code 0

上面實例用了synchronized塊，