Overview
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| Copyright 1999 Sams Publishing |
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| What Is a Thread? |
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| When a modern operating system wants to start running a program, it creates a new process. A process is a program that is currently executing. Every process has at least one thread running within it. Sometimes threads are referred to as lightweight processes . A thread is a path of code execution through a program, and each thread has its own local variables , program counter (pointer to the current instruction being executed), and lifetime. Most modern operating systems allow more than one thread to be running concurrently within a process. When the Java Virtual Machine (JavaVM, or just VM) is started by the operating system, a new process is created. Within that process, many threads can be spawned (created). |
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| Normally, you would think of Java code execution starting with the main() method and proceeding in a path through the program until all the statements in main() are completed. This is an example of a single thread. This main thread is spawned by the JavaVM, which begins execution with the main() method, executes all the statements in main() , and dies when the main() method completes. |
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| A second thread is always running in the JavaVM: the garbage collection thread. It cleans up discarded objects and reclaims their memory. Therefore, even a simple Java program that only prints Hello World to System.out is running in a multithreaded environment: The two threads are the main thread and the garbage collection thread. |
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| When a Java program includes a graphical user interface (GUI), the JavaVM automatically starts even more threads. One of these threads is in charge of delivering GUI events to methods in the program; another is responsible for painting the GUI window. |
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| For example, imagine that a GUI-based programs main thread is performing a complex and long-running calculation and that while this is going on, the user clicks a Stop Calculation button. The GUI event thread would then invoke the event handling code written for this button, allowing the calculation thread to be terminated . If only one thread was present, both of these could not be done simultaneously , and interruption would be difficult. |
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