쓰레드의 동기화

• 멀티쓰레드의 경우 여러 쓰레드가 같은 프로세스 내의 자원 공유
• 쓰레드A가 작업하던 도중 쓰레드B에게 제어권이 넘어가 공유데이터를 변경하면 의도와 다른 결과가 나온다

package com.thread.day1;

public class ThreadTest5 {

    public static void main(String[] args) {
        MyRunnable4 r = new MyRunnable4();
        Thread th1 = new Thread(r);
        Thread th2 = new Thread(r);

        th1.start();
        th2.start();

    }

}

class MyRunnable4 implements Runnable{
    int insVar = 0; //공유

    @Override
    public void run() {
        int localVar = 0;
        String name = Thread.currentThread().getName();

        while(localVar<3) {
            System.out.println(name + " local var : "+ ++localVar);
            System.out.println(name+"--------------instance var : "+ ++insVar);
            System.out.println();
        }
    }

}
/*
Thread-1 local var : 1
Thread-1--------------instance var : 1

Thread-0 local var : 1
Thread-1 local var : 2
Thread-1--------------instance var : 3

Thread-1 local var : 3
Thread-1--------------instance var : 4

Thread-0--------------instance var : 2

Thread-0 local var : 2
Thread-0--------------instance var : 5

Thread-0 local var : 3
Thread-0--------------instance var : 6

*/

package com.thread.day1;

class Circle{
    int insR=0;
}
class MyThread5 extends Thread{
    Circle c;
    MyThread5(Circle c){
        this.c=c;
    }
    public void run() {
        int localVar=0;

        while(localVar<3) {
            System.out.println(getName()+" localVar : "+ ++localVar);
            System.out.println(getName()+" --------insVar : "+ ++c.insR+"\n");
        }
    }
}
public class ThreadTest6 {

    public static void main(String[] args) {
        Circle c = new Circle();
        MyThread5 th = new MyThread5(c);
        MyThread5 th2 = new MyThread5(c);

        th.start();
        th2.start();

    }

}
/*
 * Thread-0 localVar : 1
Thread-1 localVar : 1
Thread-1 --------insVar : 2

Thread-1 localVar : 2
Thread-0 --------insVar : 1

Thread-0 localVar : 2
Thread-1 --------insVar : 3

Thread-1 localVar : 3
Thread-1 --------insVar : 5

Thread-0 --------insVar : 4

Thread-0 localVar : 3
Thread-0 --------insVar : 6

*/

• 다른 객체를 생성했지만, 같은 멤버변수를 공유

1. 동기화 (Synchronized)

• 한 번에 하나의 쓰레드만 객체에 접근할 수 있도록 객체에 락을 걸어 데이터의 일관성 유지
• 객체에 락을 걸 때

  synchronized(객체의 참조변수){}

• 메서드에 락을 걸 때

  public synchronized void func1(){}

• 한쪽에서 쓰는 동안 다른 쪽을 대기시킨다
• 공유자원을 액세스하는 문장을 synchronized로 감싸면 동시에 두 쓰레드가 하나의 객체를 액세스하지 않는다
• 먼저 작업중이던 쓰레드가 작업을 마치기 전까지는, 다른 쓰레드에게 제어권이 넘어가더라도 데이터가 변경되지 않도록 보호

synchronized()로 감싸기

package com.thread.day1;

class Account{
    int balance = 1000;

    public void withdraw(int money) {
        synchronized(this) {

            if(balance >= money) {
                try {
                    Thread.sleep(1000);
                }catch(InterruptedException e) {
                    e.printStackTrace();
                }
                balance -= money;
            }
        }
    }
}

class MyRunnable6 implements Runnable{
    Account acc = new Account();
    public void run() {
        while(acc.balance>0) {
            int money=(int)(Math.random()*3+1)*100;
            acc.withdraw(money);
            System.out.println(Thread.currentThread().getName()+":balance="+acc.balance+", money="+money);
        }
    }
}

public class SyncTest {

    public static void main(String[] args) {
        MyRunnable6 r = new MyRunnable6();
        Thread th = new Thread(r);
        Thread th2 = new Thread(r);
        th.start();
        th2.start();

    }

}

메서드에 락 걸기

package com.thread.day1;

class Account{
    int balance = 1000;

    public synchronized void withdraw(int money) {

        if(balance >= money) {
            try {
                Thread.sleep(1000);
            }catch(InterruptedException e) {
                e.printStackTrace();
            }
            balance -= money;
        }

    }
}

class MyRunnable6 implements Runnable{
    Account acc = new Account();
    public void run() {
        while(acc.balance>0) {
            int money=(int)(Math.random()*3+1)*100;
            acc.withdraw(money);
            System.out.println(Thread.currentThread().getName()+":balance="+acc.balance+", money="+money);
        }
    }
}

public class SyncTest {

    public static void main(String[] args) {
        MyRunnable6 r = new MyRunnable6();
        Thread th = new Thread(r);
        Thread th2 = new Thread(r);
        th.start();
        th2.start();

    }

}

진행바

package com.thread.day1;

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JProgressBar;
import javax.swing.JTextField;

public class ProgressBarTest extends JFrame implements ActionListener, Runnable {

    JButton bt;
    JProgressBar bar;
    JTextField tfMemo;
    JLabel lb;
    Thread th;

    public ProgressBarTest() {
        this.setLayout(new FlowLayout());

        lb = new JLabel("메모");
        tfMemo = new JTextField(30);

        add(lb);
        add(tfMemo);
        add(bt = new JButton("시작"));
        bar = new JProgressBar();
        bar.setStringPainted(true); // 진행바에 퍼센티지 표시
        add(bar);

        bt.addActionListener(this);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(400,300);
        setVisible(true);
    }

    public static void main(String[] args) {
        new ProgressBarTest();

    }

    @Override
    public void run() {
        synchronized(bar) {
            System.out.println(bar.getMinimum()+ " " +bar.getMaximum());

            for (int i = (bar.getMinimum());i<=bar.getMaximum(); i+=5) {
                System.out.println("i="+i);
                try {
                    Thread.sleep(200);
                }catch (InterruptedException e) {
                    e.printStackTrace();
                }
                bar.setValue(i);
            }
        }

    }

    @Override
    public void actionPerformed(ActionEvent e) {
        th = new Thread(this);
        th.start(); //runnable로 진입시킨다

    }

}