A Java Developer’s Guide to Threads on iPhone

Java makes it ridiculously simple to run code in threads and ensure that your objects are thread safe. When developing user experiences, threads enable you to do things in the “background” (like downloading and parsing content) without rendering your application unresponsive and giving the user the sense that the application may have “frozen”. The value of threads apply just a much to an iPhone application.

In Java, there are three key things you need to know about to work effectively with threads …

1. java.lang.Thread
2. java.lang.Runnable
3. synchronized (…) { … }

… interestingly, in Objective-C there are almost the same constructs …

1. NSThread
2. @synchronized (…) { … }

The key difference is the absence of the “Runnable” interface. This may be misleading at first until you recognize that Objective-C frameworks often don’t declare formal protocols for their delegates … in fact, using Objective-C, you can pass in the method to be executed by the thread without it having to be wrapped in a formal class.

Let’s consider a simple example in Java. First you would begin by defining a class that implemented the Runnable interface.

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public class MyClass implements Runnable {
 
    public void startMyThread() {
      Thread myThread = new Thread(this);
      myThread.start();
    }
    public void run() {
        /* Do your threaded code here */
    }
}

In Objective-C we would do this all like this …

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@implementation 
- (void) startMyThread {
  [NSThread detachNewThreadSelector:@selector(run) toTarget:self withObject:nil];
}
 
- (void) run {
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
 
   /* Do your threaded code here */
 
  [pool release];
} 
@end

Some of the things to note about the Objective-C side of this are:

• You must always specify a NSAutoreleasePool and release it for each thread you run
• You can pass any method to NSThread, it doesn’t have to be called “run”
• You can even pass in parameters to the “run” method unlike the Java equivalent

There is a lot more to learn about NSThread and I encourage you to take a look. In my opinion it has some features that are better than the Java equivalent. In a future post I’ll cover off on how to make you classes thread-safe using the @synchronized() {..} block. For those of you already familiar with Java, @synchronized is exactly like its Java counter part.