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This program asks the user if he or she wishes the other thread to be a daemon thread. The user types in a string. If that string starts with a Y the other thread is set to be a daemon thread. Here s an example of the output when the user types a Y:
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Message isAlive isDaemon Before Start false false Other thread is Daemon [Y/N] Y * After Start true true Main thread exiting *
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Because of timing issues the main thread exits first. The key element is that when the main thread terminates the background thread is stopped. The following shows what happens when the user types in N:
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Message isAlive isDaemon Before Start false false Other thread is Daemon [Y/N] N * After Start true false Main thread exiting * * * *
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Notice that the other thread continues to execute after the main thread has exited. The reason is that both the main thread and the other thread are foreground threads. Suspending and resuming a thread In chapter 5 we discussed suspending and resuming threads. J# s versions of suspend and resume function much like their System.Threading.Thread counterparts. The only noticeable difference is that the J# methods do not raise exceptions when they are called from an incorrect state. For example, if a thread is not in the suspended state and the framework s resume is called, a ThreadStateException is raised. The J# methods do not raise exceptions based on thread state. Listing 18.10 contains the listing of a console application that allows the user to suspend and resume the other thread.
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Listing 18.10 Allowing the user to suspend and resume the other thread interactively (J#)
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package PauseAndResume; import System.Console; public class ClassMain { /** @attribute System.STAThread() */
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public static void main(String[] args) throws Exception { OtherThread t; t=new OtherThread(100); t.setDaemon(true); t.start(); Console.WriteLine("q = Quit"); Console.WriteLine("s = Suspend"); Console.WriteLine("r = Resume"); boolean keepGoing = true; while (keepGoing) { String input = Console.ReadLine(); char inputChar = input.toLowerCase().charAt(0); switch(inputChar) { case 'q': keepGoing =false; break; case 's': t.suspend(); break; case 'r': t.resume(); break; } } } }
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The following output shows that an exception isn t raised when the thread is running and resume is called:
q = Quit s = Suspend r = Resume * * * r* * r *
Notice that the other thread is executing and resume is signaled. The suspend and resume methods are deprecated, meaning they will likely be removed from J# in future releases. Rather than having an external thread control the execution of a thread, it s better to have the thread itself control it using the wait and sleep statements. This removes the possibility that the thread is suspended at a point where it has a resource allocated, such as a synchronized region of code. J# S THREAD CLASS 313
THE RUNNABLE INTERFACE
J#, along with C# and VB.NET, allows for inheritance from only one class. This single inheritance restriction greatly simplifies object-oriented development. There are times that it is desirable for a class to contain multiple types of reusable functionality. This is where the concept of an interface comes in. An interface is nothing more than a way of stating what methods and properties an object must implement if it claims to support an interface. The Runnable interface in J# is used to create threads without deriving from the Thread class. If an object implements the Runnable interface it must contain a method named run that accepts no parameters and does not return a value. The instance of the object that supports the Runnable interface is passed to the constructor of the Thread class. The instance of the Thread class can then be used to start the thread, which will begin executing the run method. Listing 18.11 contains a base class that provides an Output method. This is intended to serve as an example of the need for interfaces, not to demonstrate object-oriented design. This class is overly simple in the hope that it will make the concepts clearer.
Listing 18.11 A very simple base class (J#)
package SimpleRunnable; import System.Console; public class BaseClass { protected void Output(String message) { Console.WriteLine(message); } }
Listing 18.12 contains a class that is derived from the BaseClass class. Since the DerivedClass class extends the BaseClass class, it cannot also extend the Thread class. Instead, it implements the Runnable interface.
Listing 18.12 The DerivedClass implements the Runnable interface (J#).
package SimpleRunnable; public class DerivedClass extends BaseClass implements Runnable { public void run() { Output("Enter Second thread"); try { Output(Thread.currentThread().getName()); } catch(Exception ex) {
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Output(ex.getMessage()); } for (int i=0;i<4;i++) { Output("*"); } Output("Exit Second thread"); } }
Because the DerivedClass claims to implement the Runnable interface it must contain a function named run. The run function is the entry point for the new thread, just as it is when the class is derived from the Thread class. The run function is not called directly, but is invoked as a result of the start method being invoked on an instance of the Thread class. Listing 18.13 contains the source code of a console application that creates an instance of the DerivedClass, associates it with an instance of the Thread class, and starts the new thread using the start method.
Listing 18.13 Allocates an instance of the Thread class and starts the new thread (J#)
package SimpleRunnable; import System.Console; public class ClassMain { /** @attribute System.STAThread() */ public static void main(String[] args) { Console.WriteLine("Enter main thread"); Implements DerivedClass derived; Runnable derived = new DerivedClass(); Is used to start Thread theNewThread; the new thread theNewThread= new Thread(derived); theNewThread.setName("SecondThread"); Console.WriteLine("Starting second thread"); Starts the theNewThread.start(); new thread try { Console.WriteLine("Main thread is joining second thread"); Waits until the run theNewThread.join(); method terminates } catch(InterruptedException ex) { } Console.WriteLine("Exit Main thread"); } }
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