Master Java’s Parent Delegation Model: How Class Loaders Work and Why It Matters

What Is the Parent Delegation Model

The parent delegation model means that when loading a class, the request is first delegated to its parent class loader; only if the parent cannot load the class does the current loader attempt to load it.

Parent Delegation Model Diagram

Here “parent” refers to the upward‑then‑downward lookup process that defines the delegation model.

Advantages and Disadvantages of the Parent Delegation Model

Advantages

The model has two main benefits:

1. Avoid duplicate class loading

When a class loader needs a class, it first delegates to its parent; only if the parent cannot find the class does it load it itself, preventing the same class from being loaded multiple times.

2. Ensure class safety

The model protects core libraries by preventing user‑defined classes from overriding core classes, reducing the risk of malicious code. For example, the JRE’s String class is loaded by the bootstrap loader, so a user‑defined unsafe String class will not be loaded.

Disadvantages

Because of the loading scope limitation, a top‑level ClassLoader cannot access classes loaded by lower‑level ClassLoaders.

Steps of the Parent Delegation Model

The Java parent delegation model can be summarized in six steps, illustrated below:

1. When loading a class, the application class loader’s cache is checked first; if found, the class is returned, otherwise the process continues.

2. The extension class loader’s cache is checked; if found, the class is returned, otherwise continue.

3. The bootstrap class loader is queried; if found, the class is returned, otherwise continue.

4. The extension class loader attempts to find and load the class; if successful, the class is cached and returned.

5. The application class loader attempts to find and load the class; if successful, the class is cached and returned; otherwise a ClassNotFound exception is thrown.

6. Once a class loader loads a class, it caches the class definition for future requests.

These steps provide a comprehensive understanding of the parent delegation model.