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Building web applications are more complicated than monolithic applications, largely because of multithreading. Your code will be servicing many different users simultaneously across many different threads. This tends to complicate the code you write, since some fundamental aspects of object oriented development get called into question: in particular, the use of internal state, values stored inside instance variables, since in a multi-threaded multithreaded environment, that's no longer the safe place it is in traditional development. Shared objects plus internal state plus multiple threads equals an broken, unpredictable application.

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In any case, injection "just happens". Tapestry finds the constructor of your class and analyzes the parameters to determine what to pass in. In some cases, it uses just the parameter type to find a match, in other cases, annotations on the parameters may also be used. It also scans through the fields of your service implementation class to identify which should have injected values written into them.

Why can't I just use new?

I've had this question asked me many a timeThat's a common question. All these new concepts seem alien. All that XML (in the Spring or HiveMind IoC containers; Tapestry IoC uses no XML) is a burden. What's wrong with new?

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This example is built around some work I've done recently involving real-world work that involves a Java Messaging Service queue, part of an application performance monitoring subsystem for a large application. Code inside each server collects performance data of various types and sends it, via a shared JMS queue, to a central server for collection and reporting.

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public class TableMetricProducer implements MetricProducer
{
  . . . 

  public void execute() 
  {

    int rowCount = . . .;
    Metric metric = new Metric("app/clients", System.currentTimeMillis(), rowCount);

    new QueueWriter().sendMetric(metric);
  }
}

IWe've elided omitted some of the details (this code will need a database URL or a connection pool to operate), so as to focus on the one method and it's relationship to the QueueWriter class.

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public class MonitorModule
{
  public static void bind(ServiceBinder binder)
  {
    binder.bind(QueueWriter.class, QueueWriterImpl.class);
    binder.bind(MetricScheduler.class, MetricSchedulerImpl.class);
  }

  public void contributeMetricScheduler(Configuration<MetricProducer> configuration, QueueWriter queueWriter, . . .)
  {
    configuration.add(new TableMetricProducer(queueWriter, . . .))
  }
}

Again, Iwe've elided out omitted a few details related to the database the TableMetricProducer will point at (in fact, Tapestry IoC provides a lot of support for configuration of this type as well, which is yet another concern).

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Over a decade ago, when Java first came on the scene, it was the first mainstream language to support garbage collection. This was very controversial: the garbage collector was seen as unnecessary, and a waste of resources. Among C and C++ developers, the attitude was "Why do I need a garbage collector? If I call malloc() I can call free()."

I don't know about you, but I don't think I could ever But now, most developers would never want to go back to a non-garbage collected environment. Having the GC around makes it much easier to code in a way I we find natural: many small related objects working together. It turns out that knowing when to call free() is more difficult than it sounds. The Objective-C language tried to solve this with retain counts on objects and that still lead to memory leaks when it was applied to object graphs rather than object trees.

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