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Java Reflection - Classes

Jakob Jenkov
Last update: 2014-05-21

Using Java Reflection you can inspect Java classes at runtime. Inspecting classes is often the first thing you do when using Reflection. From the classes you can obtain information about

plus a lot more information related to Java classes. For a full list you should consult the JavaDoc for java.lang.Class. This text will briefly touch upon all accessing of the above mentioned information. Some of the topics will also be examined in greater detail in separate texts. For instance, this text will show you how to obtain all methods or a specific method, but a separate text will show you how to invoke that method, how to find the method matching a given set of arguments if more than one method exists with the same name, what exceptions are thrown from method invocation via reflection, how to spot a getter/setter etc. The purpose of this text is primarily to introduce the Class object and the information you can obtain from it.

The Class Object

Before you can do any inspection on a class you need to obtain its java.lang.Class object. All types in Java including the primitive types (int, long, float etc.) including arrays have an associated Class object. If you know the name of the class at compile time you can obtain a Class object like this:

    Class myObjectClass = MyObject.class

If you don't know the name at compile time, but have the class name as a string at runtime, you can do like this:

String className = ... //obtain class name as string at runtime Class class = Class.forName(className);

When using the Class.forName() method you must supply the fully qualified class name. That is the class name including all package names. For instance, if MyObject is located in package com.jenkov.myapp then the fully qualified class name is com.jenkov.myapp.MyObject

The Class.forName() method may throw a ClassNotFoundException if the class cannot be found on the classpath at runtime.

Class Name

From a Class object you can obtain its name in two versions. The fully qualified class name (including package name) is obtained using the getName() method like this:

    Class aClass = ... //obtain Class object. See prev. section
    String className = aClass.getName();

If you want the class name without the pacakge name you can obtain it using the getSimpleName() method, like this:

    Class  aClass          = ... //obtain Class object. See prev. section
    String simpleClassName = aClass.getSimpleName();

Modifiers

You can access the modifiers of a class via the Class object. The class modifiers are the keywords "public", "private", "static" etc. You obtain the class modifiers like this:

  Class  aClass = ... //obtain Class object. See prev. section
  int modifiers = aClass.getModifiers();

The modifiers are packed into an int where each modifier is a flag bit that is either set or cleared. You can check the modifiers using these methods in the class java.lang.reflect.Modifier:

    Modifier.isAbstract(int modifiers)
    Modifier.isFinal(int modifiers)
    Modifier.isInterface(int modifiers)
    Modifier.isNative(int modifiers)
    Modifier.isPrivate(int modifiers)
    Modifier.isProtected(int modifiers)
    Modifier.isPublic(int modifiers)
    Modifier.isStatic(int modifiers)
    Modifier.isStrict(int modifiers)
    Modifier.isSynchronized(int modifiers)
    Modifier.isTransient(int modifiers)
    Modifier.isVolatile(int modifiers)

Package Info

You can obtain information about the package from a Class object like this:

Class  aClass = ... //obtain Class object. See prev. section
Package package = aClass.getPackage();

From the Package object you have access to information about the package like its name. You can also access information specified for this package in the Manifest file of the JAR file this package is located in on the classpath. For instance, you can specify package version numbers in the Manifest file. You can read more about the Package class here: java.lang.Package

Superclass

From the Class object you can access the superclass of the class. Here is how:

Class superclass = aClass.getSuperclass();

The superclass class object is a Class object like any other, so you can continue doing class reflection on that too.

Implemented Interfaces

It is possible to get a list of the interfaces implemented by a given class. Here is how:

Class  aClass = ... //obtain Class object. See prev. section
Class[] interfaces = aClass.getInterfaces();

A class can implement many interfaces. Therefore an array of Class is returned. Interfaces are also represented by Class objects in Java Reflection.

NOTE: Only the interfaces specifically declared implemented by a given class is returned. If a superclass of the class implements an interface, but the class doesn't specifically state that it also implements that interface, that interface will not be returned in the array. Even if the class in practice implements that interface, because the superclass does.

To get a complete list of the interfaces implemented by a given class you will have to consult both the class and its superclasses recursively.

Constructors

You can access the constructors of a class like this:

 Constructor[] constructors = aClass.getConstructors();

Constructors are covered in more detail in the text on Constructors.

Methods

You can access the methods of a class like this:

 Method[] method = aClass.getMethods();

Methods are covered in more detail in the text on Methods.

Fields

You can access the fields (member variables) of a class like this:

 Field[] method = aClass.getFields();

Fields are covered in more detail in the text on Fields.

Annotations

You can access the class annotations of a class like this:

 Annotation[] annotations = aClass.getAnnotations();

Annotations are covered in more detail in the text on Annotations.

Jakob Jenkov




Copyright  Jenkov Aps
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