Question

Why Appletviewer throws error with this:

thread = new Thread(this,”main thread”);

Answer 1

I suspect you Thread is trying to do one of these in your Thread:

• Read files on the client file system.
• Write files to the client file
  system.
• Delete files on the client file
  system, either by using the
  File.delete() method, or by calling
  system-level rm or del commands.
• Rename files on the client file
  system, either by using the
  File.renameTo() method, or by calling
  system-level mv or rename commands.
• Create a directory on the client file
  system, either by using the
  File.mkdirs() methods or by calling
  the system-level mkdir command.
• List the contents of a directory.
• Check to see whether a file exists.
• Obtain information about a file,
  including size, type, and
  modification timestamp.
• Create a network connection to any
  computer other than the host from
  which it originated.
• Listen for or accept network
  connections on any port on the client
  system.
• Create a top-level window without an
  untrusted window banner.
• Obtain the user’s username or home
  directory name through any means,
  including trying to read the system
  properties: user.name, user.home,
  user.dir, java.home, and
  java.class.path.
• Define any system properties.
• Run any program on the client system
  using the Runtime.exec() methods.
• Make the Java interpreter exit, using
  either System.exit() or
  Runtime.exit().
• Load dynamic libraries on the client
  system using the load() or
  loadLibrary() methods of the Runtime
  or System classes.
• Create or manipulate any thread that
  is not part of the same ThreadGroup
  as the applet.
• Create a ClassLoader.
• Create a SecurityManager.
• Specify any network control
  functions, including
  ContentHandlerFactory,
  SocketImplFactory, or
  URLStreamHandlerFactory.
• Define classes that are part of
  packages on the client system.

You have two solutions

1. Digitally sign your applet
2. Change the security permissions in
   your JRE

See this for security and applets