java.security.AccessControlException：运行applet时发生

Question

我正在从Herbert Schildt Book Java2 Complete Reference Fifth Edition中逐步学习Java。在我创建简单的Banner Applet的过程中，它通过创建Thread并调用Applet的Repaint（）方法来显示横幅并在Applet Viewer上滚动它。但是在创建可运行Target的线程对象时，会抛出这样的异常

java.security.AccessControlException: access denied (java.lang.RuntimePermission modifyThreadGroup)
at java.security.AccessControlContext.checkPermission(AccessControlContext.java:345)
at java.security.AccessController.checkPermission(AccessController.java:555)
at java.lang.SecurityManager.checkPermission(SecurityManager.java:549)
at sun.applet.AppletSecurity.checkAccess(AppletSecurity.java:252)
at java.lang.ThreadGroup.checkAccess(ThreadGroup.java:304)
at java.lang.ThreadGroup.<init>(ThreadGroup.java:119)
at java.lang.ThreadGroup.<init>(ThreadGroup.java:95)
at Applet.SimpleBanner.start(SimpleBanner.java:49)
at sun.applet.AppletPanel.run(AppletPanel.java:475)
at java.lang.Thread.run(Thread.java:713)

我读了其他文章说它需要安全许可但在我的applet.policy文件中已经允许所有权限

grant { permission java.security.AllPermission;};

这只是我的第二个小程序。任何人都可以用简单的术语详细解释为什么它是Throwing Security Exception及其解决方案？

这是我的小程序代码。

import java.applet.Applet;
import java.awt.*;

/* A Simple Banner Applet.
 * This Banner Applet Creates a thread that scrolls the message contained
 * in msg right to left across banner's window.
 */

/*
 * <applet code="SimpleBanner" width=300 height=50>
 * </applet>
 */

public class SimpleBanner extends Applet implements Runnable{
private static final long serialVersionUID = 1L;
String msg = "Hello World";
Thread t = null;
ThreadGroup Grp;
int state;
boolean stopflag;

/**
 * Initialization method that will be called after the applet is loaded into
 * the browser.
 */
@Override
public void init() 
    {
    //Set Foreground and background color
    setBackground(Color.cyan);
    setForeground(Color.red);
    }

@Override
public void start()
    {
    //Start Thread
    Grp = new ThreadGroup("Group");
    t = new Thread(Grp, this);
    stopflag = false;
    t.start();
    }
//Entry Point for Thread that Runs The banner
@Override
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 ex) {

        }
    }
}

@Override
public void stop()
{
// Pause The Banner
stopflag=true;
t=null;
}

@Override
public void paint(Graphics g)
{
//Display The Banner
    g.drawString(msg, 50, 30);
}
}

Answer 1

默认情况下，由于安全原因，Applet在具有受限权限的沙箱环境中运行。小程序没有运行时权限来创建或修改线程组，因此您将获得异常。不要创建新的线程组。或者覆盖您的安全策略，通过授予运行时创建或修改线程组的权限，明确允许您的applet创建一个。要覆盖默认权限，请在用户主目录的.java.policy文件中定义相应的策略。建议您编辑特定于用户的策略文件，而不是JRE安全目录下的全局策略文件。

使用JDK的策略工具来定义策略或手动执行。请参阅以下模板：

grant codeBase "<code base>" {
  permission <type> "<target>", "<actions>";
  permission <type> "<target>", "<actions>";
  ...
};

For eg. 
grant codeBase "http://geosim.cs.vt.edu/geosim/-" {
  permission java.lang.RuntimePermission "modifyThreadGroup";
  ...
};