java:是线程中可见的全局变量 [英] java: are global variables visible in threads
问题描述
如果我在主线程中声明一个全局变量,假设从主线程运行一个新线程,新线程是否可以访问主线程中的全局变量?
msg字符串是我的变量
/ *一个简单的横幅applet。
这个小程序创建一个线程,用于在msg的右边滚动
消息,使其在applet的窗口中向右滚动
。
* /
import java.awt。*;
import java.applet。*;
/ *
< / applet>
* /
公共类AppletSkel扩展了Applet实现的Runnable {
String msg =一个简单的移动横幅。 //<< -----------------访问
的变量Thread t = null;
int状态;
boolean stopFlag;
//设置颜色并初始化线程。
public void init(){
setBackground(Color.cyan);
setForeground(Color.red);
//启动线程
public void start(){
t = new Thread(this);
stopFlag = false;
t.start();
}
//运行横幅的线程的入口点。
public void run(){
char ch;
//显示横幅
((;;)){
try {
repaint();
Thread.sleep(250);
ch = msg.charAt(0);
msg = msg.substring(1,msg.length());
msg + = ch;
if(stopFlag)
break;
} catch(InterruptedException e){}
}
}
//暂停横幅。
public void stop(){
stopFlag = true;
t = null;
}
//显示横幅。
public void paint(Graphics g){
g.drawString(msg,50,30);
g.drawString(msg,80,40);
$ div $解析方案
变量对于多个线程可见的通常是棘手的。但是,字符串是不可变的,因此简化了情况。
它是可见的,但是当其他线程可用的修改的普通值不能保证时。你应该使它 volatile
,这样它就不会在本地缓存线程。在分配 msg
之前使用局部变量来构建新字符串。
stopFlag
来自其他线程,它也应该是 volatile
。 if I declare a global variable in the main thread, suppose that from the main thread I run a new thread, can the new thread access the global variable in the main thread?
"msg" string is my variable to acces
/* A simple banner applet.
This applet creates a thread that scrolls
the message contained in msg right to left
across the applet's window.
*/
import java.awt.*;
import java.applet.*;
/*
<applet code="SimpleBanner" width=300 height=50>
</applet>
*/
public class AppletSkel extends Applet implements Runnable {
String msg = " A Simple Moving Banner."; //<<-----------------VARIABLE TO ACCESS
Thread t = null;
int state;
boolean stopFlag;
// Set colors and initialize thread.
public void init() {
setBackground(Color.cyan);
setForeground(Color.red);
}
// Start thread
public void start() {
t = new Thread(this);
stopFlag = false;
t.start();
}
// Entry point for the thread that runs the banner.
public void run() {
char ch;
// Display banner
for( ; ; ) {
try {
repaint();
Thread.sleep(250);
ch = msg.charAt(0);
msg = msg.substring(1, msg.length());
msg += ch;
if(stopFlag)
break;
} catch(InterruptedException e) {}
}
}
// Pause the banner.
public void stop() {
stopFlag = true;
t = null;
}
// Display the banner.
public void paint(Graphics g) {
g.drawString(msg, 50, 30);
g.drawString(msg, 80, 40);
}
}
Variables that are visible to several threads are generally tricky. Strings, however, are immutable, so that simplifies the situation.
It is visible, but when a modified ordinary value is available to other threads is not guaranteed. You should make it volatile
, so that it is not cached thread locally. Use a local variable to build the new string before assigning msg
.
If you intend to modify stopFlag
from other threads, it should also be volatile
.
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