Symptoms

Classes that depend on the container.

Consider the following example. We have a class BImpl that requires an A instance. It declares the dependency on the container so it can look up that A:

public interface A {
}

public class AImpl implements A {
}

public class BImpl implements B {
    private final A a;

    BImpl(PicoContainer pico) {
        a = (A) pico.getComponentOfType(A.class);
        
        /*
        alternatively:
        a = (A) pico.getComponent("a");
        */
    }
}

It can be used in the following way:

MutablePicoContainer pico = new DefaultPicoContainer();
pico.addComponent("a", AImpl.class);
pico.addComponent("b", BImpl.class);
pico.addComponent(pico);

...
B b = (B) pico.getComponent("b");

This will work, but it's an antipattern.

The reasons why the above implementation of BImpl is an antipattern are:

• It introduces an unneeded dependency from BImpl to the container.
• This makes BImpl hard to test without a container.
• B assumes that the container has a registered an A. As a result, B won't FailFast if it hasn't. Instead, a will reference null, and BImpl will FailLate.

Causes

Not sure. Poor understanding of how PicoContainer works? Not being able to think "simple"?

What to do

The simple and elegant solution to this antipattern is not to complicate the world more than it is.

Here is how it should be:

public class BImpl implements B {
    private final A a;

    BImpl(A a) {
        this.a = a;
    }
}

PicoContainer will figure out that BImpl needs an A instance, and will pass in the AImpl, as this is an implementation of A.