Achieve strong performance with threads, Part 1

Introducing threads, the Thread class, and runnables

By Jeff Friesen, JavaWorld.com, 05/03/02

Users hate unresponsive software. When users click a mouse, they expect a program to instantly respond to their requests, even when the program is in the midst of a time-consuming activity, such as repaginating a long document or waiting for a network operation to complete. Programs that respond slowly to their users exhibit poor performance. To improve a program's performance, developers typically use threads.

This article is the first in a four-part series that explores threads. Although you might think threads a difficult subject to grasp, I intend to show you that threads are easy to understand. In this article, I introduce you to threads and the Thread class, and discuss runnables. Furthermore, the accompanying sidebar, "Exceptions and the run() Method" discusses exceptions in the context of a thread object's run() method. In subsequent articles, I will explore synchronization (via locks), synchronization problems (such as deadlock), the wait/notify mechanism, scheduling (with and without priority), thread interruption, timers, volatility, thread groups, and thread local variables.

Read the whole series on thread programming:

• Part 1: Introducing threads, the Thread class, and runnables
• Part 2: Use synchronization to serialize thread access to critical code sections
• Part 3: Learn about thread scheduling, the wait/notify mechanism, and thread interruption
• Part 4: Discover thread groups, volatility, thread-local variables, timers, and the ThreadDeath class

What is a thread?

Conceptually, the notion of a thread is not difficult to grasp: it's an independent path of execution through program code. When multiple threads execute, one thread's path through the same code usually differs from the others. For example, suppose one thread executes the byte code equivalent of an if-else statement's if part, while another thread executes the byte code equivalent of the else part. How does the JVM keep track of each thread's execution? The JVM gives each thread its own method-call stack. In addition to tracking the current byte code instruction, the method-call stack tracks local variables, parameters the JVM passes to a method, and the method's return value.

When multiple threads execute byte-code instruction sequences in the same program, that action is known as multithreading. Multithreading benefits a program in various ways:

• Multithreaded GUI (graphical user interface)-based programs remain responsive to users while performing other tasks, such as repaginating or printing a document.
• Threaded programs typically finish faster than their nonthreaded counterparts. This is especially true of threads running on a multiprocessor machine, where each thread has its own processor.

Java accomplishes multithreading through its java.lang.Thread class. Each Thread object describes a single thread of execution. That execution occurs in Thread's run() method. Because the default run() method does nothing, you must subclass Thread and override run() to accomplish useful work. For a taste of threads and multithreading in the context of Thread, examine Listing 1:

Listing 1. ThreadDemo.java

// ThreadDemo.java
class ThreadDemo
{
   public static void main (String [] args)
   {
      MyThread mt = new MyThread ();
      mt.start ();
      for (int i = 0; i < 50; i++)
           System.out.println ("i = " + i + ", i * i = " + i * i);
   }
}
class MyThread extends Thread
{
   public void run ()
   {
      for (int count = 1, row = 1; row < 20; row++, count++)
      {
           for (int i = 0; i < count; i++)
                System.out.print ('*');
           System.out.print ('\n');
      }
   }
}

Listing 1 presents source code to an application consisting of classes ThreadDemo and MyThread. Class ThreadDemo drives the application by creating a MyThread object, starting a thread that associates with that object, and executing some code to print a table of squares. In contrast, MyThread overrides Thread's run() method to print (on the standard output stream) a right-angle triangle composed of asterisk characters.

When you type java ThreadDemo to run the application, the JVM creates a starting thread of execution, which executes the main() method. By executing mt.start ();, the starting thread tells the JVM to create a second thread of execution that executes the byte code instructions comprising the MyThread object's run() method. When the start() method returns, the starting thread executes its for loop to print a table of squares, while the new thread executes the run() method to print the right-angle triangle.

What does the output look like? Run ThreadDemo to find out. You will notice each thread's output tends to intersperse with the other's output. That results because both threads send their output to the same standard output stream.

The Thread class

To grow proficient at writing multithreaded code, you must first understand the various methods that make up the Thread class. This section explores many of those methods. Specifically, you learn about methods for starting threads, naming threads, putting threads to sleep, determining whether a thread is alive, joining one thread to another thread, and enumerating all active threads in the current thread's thread group and subgroups. I also discuss Thread's debugging aids and user threads versus daemon threads.

I'll present the remainder of Thread's methods in subsequent articles, with the exception of Sun's deprecated methods.

Construct threads

Thread has eight constructors. The simplest are:

• Thread(), which creates a Thread object with a default name
• Thread(String name), which creates a Thread object with a name that the name argument specifies

The next simplest constructors are Thread(Runnable target) and Thread(Runnable target, String name). Apart from the Runnable parameters, those constructors are identical to the aforementioned constructors. The difference: The Runnable parameters identify objects outside Thread that provide the run() methods. (You learn about Runnable later in this article.) The final four constructors resemble Thread(String name), Thread(Runnable target), and Thread(Runnable target, String name); however, the final constructors also include a ThreadGroup argument for organizational purposes.