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Java - Reflection API
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  Introduction

Basics

The Reflection API allows one to check the class structure of the code at runtime and invoke code dynamically. This is very powerful, but it is also dangerous since the compiler is not able to statically determine whether dynamic invocations are valid.

A simple example would be to get the public constructors and methods of a given class:

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

// This is a object representing the String class (not an instance of String!)
Class<String> clazz = String.class;

Constructor<?>[] constructors = clazz.getConstructors(); // returns all public constructors of String
Method[] methods = clazz.getMethods(); // returns all public methods from String and parents

With this information it is possible to instance the object and call different methods dynamically.

Reflection and Generic Types

Generic type information is available for:

  • method parameters, using getGenericParameterTypes().
  • method return types, using getGenericReturnType().
  • public fields, using getGenericType.

The following example shows how to extract the generic type information in all three cases:

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

public class GenericTest {

    public static void main(final String[] args) throws Exception {
        final Method method = GenericTest.class.getMethod("testMethod", Map.class);
        final Field field = GenericTest.class.getField("testField");

        System.out.println("Method parameter:");
        final Type parameterType = method.getGenericParameterTypes()[0];
        displayGenericType(parameterType, "\t");

        System.out.println("Method return type:");
        final Type returnType = method.getGenericReturnType();
        displayGenericType(returnType, "\t");

        System.out.println("Field type:");
        final Type fieldType = field.getGenericType();
        displayGenericType(fieldType, "\t");

    }

    private static void displayGenericType(final Type type, final String prefix) {
        System.out.println(prefix + type.getTypeName());
        if (type instanceof ParameterizedType) {
            for (final Type subtype : ((ParameterizedType) type).getActualTypeArguments()) {
                displayGenericType(subtype, prefix + "\t");
            }
        }

    }

    public Map<String, Map<Integer, List<String>>> testField;

    public List<Number> testMethod(final Map<String, Double> arg) {
        return null;
    }

}

This results in the following output:

Method parameter:
    java.util.Map<java.lang.String, java.lang.Double>
        java.lang.String
        java.lang.Double
Method return type:
    java.util.List<java.lang.Number>
        java.lang.Number
Field type:
    java.util.Map<java.lang.String, java.util.Map<java.lang.Integer, java.util.List<java.lang.String>>>
        java.lang.String
        java.util.Map<java.lang.Integer, java.util.List<java.lang.String>>
            java.lang.Integer
            java.util.List<java.lang.String>
                java.lang.String
  Dynamic Proxies

Dynamic Proxies do not really have much to do with Reflection but they are part of the API. It's basically a way to create a dynamic implementation of an interface. This could be helpful when creating mockup services.
A Dynamic Proxy is an instance of an interface that is created with a so-called invocation handler that intercepts all method calls and allows the handling of their invocation manually.

public class DynamicProxyTest {

    public interface MyInterface1{
        public void someMethod1();
        public int someMethod2(String s);
    }

    public interface MyInterface2{
        public void anotherMethod();
    }
   
    public static void main(String args[]) throws Exception {
        // the dynamic proxy class 
        Class<?> proxyClass = Proxy.getProxyClass(
                ClassLoader.getSystemClassLoader(),
                new Class[] {MyInterface1.class, MyInterface2.class});
        // the dynamic proxy class constructor
        Constructor<?> proxyConstructor = 
            proxyClass.getConstructor(InvocationHandler.class);

        // the invocation handler
        InvocationHandler handler = new InvocationHandler(){
            // this method is invoked for every proxy method call
            // method is the invoked method, args holds the method parameters
            // it must return the method result
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
                String methodName = method.getName();

                if(methodName.equals("someMethod1")){
                    System.out.println("someMethod1 was invoked!");
                    return null;
                }
                if(methodName.equals("someMethod2")){
                    System.out.println("someMethod2 was invoked!");
                    System.out.println("Parameter: " + args[0]);
                    return 42;
                }
                if(methodName.equals("anotherMethod")){
                    System.out.println("anotherMethod was invoked!");
                    return null;
                }
                System.out.println("Unkown method!");
                return null;
            }
        };

        // create the dynamic proxy instances
        MyInterface1 i1 = (MyInterface1) proxyConstructor.newInstance(handler);
        MyInterface2 i2 = (MyInterface2) proxyConstructor.newInstance(handler);

        // and invoke some methods
        i1.someMethod1();
        i1.someMethod2("stackoverflow");
        i2.anotherMethod();
    }
}

The result of this code is this:

someMethod1 was invoked!
someMethod2 was invoked!
Parameter: stackoverflow
anotherMethod was invoked!
  Getting and Setting fields

Using the Reflection API, it is possible to change or get the value of a field at runtime. For example, you could use it in an API to retrieve different fields based on a factor, like the OS. You can also remove modifiers like final to allow modifing fields that are final.

To do so, you will need to use the method Class#getField() (opens new window) in a way such as the one shown below:

// Get the field in class SomeClass "NAME".
Field nameField = SomeClass.class.getDeclaredField("NAME");

// Get the field in class Field "modifiers". Note that it does not 
// need to be static
Field modifiersField = Field.class.getDeclaredField("modifiers");

// Allow access from anyone even if it's declared private
modifiersField.setAccessible(true);

// Get the modifiers on the "NAME" field as an int.
int existingModifiersOnNameField = nameField.getModifiers();

// Bitwise AND NOT Modifier.FINAL (16) on the existing modifiers
// Readup here https://en.wikipedia.org/wiki/Bitwise_operations_in_C
// if you're unsure what bitwise operations are.
int newModifiersOnNameField = existingModifiersOnNameField & ~Modifier.FINAL;

// Set the value of the modifiers field under an object for non-static fields
modifiersField.setInt(nameField, newModifiersOnNameField);

// Set it to be accessible. This overrides normal Java 
// private/protected/package/etc access control checks.
nameField.setAccessible(true);

 // Set the value of "NAME" here. Note the null argument. 
 // Pass null when modifying static fields, as there is no instance object
nameField.set(null, "Hacked by reflection...");

// Here I can directly access it. If needed, use reflection to get it. (Below)
System.out.println(SomeClass.NAME);

Getting fields is much easier. We can use Field#get() (opens new window) and its variants to get its value:

// Get the field in class SomeClass "NAME".
Field nameField = SomeClass.class.getDeclaredField("NAME");

// Set accessible for private fields
nameField.setAccessible(true);

// Pass null as there is no instance, remember?
String name = (String) nameField.get(null);

Do note this:

When using Class#getDeclaredField (opens new window), use it to get a field in the class itself:

class HackMe extends Hacked {
    public String iAmDeclared;
}

class Hacked {
    public String someState;
}

Here, HackMe#iAmDeclared is declared field. However, HackMe#someState is not a declared field as it is inherited from its superclass, Hacked.
  Misuse of Reflection API to change private and final variables

Reflection is useful when it is properly used for right purpose. By using reflection, you can access private variables and re-initialize final variables.

Below is the code snippet, which is not recommended.

import java.lang.reflect.*;

public class ReflectionDemo{
    public static void main(String args[]){
        try{
            Field[] fields = A.class.getDeclaredFields();
            A a = new A();
            for ( Field field:fields ) {
                if(field.getName().equalsIgnoreCase("name")){
                    field.setAccessible(true);
                    field.set(a, "StackOverFlow");
                    System.out.println("A.name="+field.get(a));
                }
                if(field.getName().equalsIgnoreCase("age")){
                    field.set(a, 20);
                    System.out.println("A.age="+field.get(a));
                }
                if(field.getName().equalsIgnoreCase("rep")){
                    field.setAccessible(true);
                    field.set(a,"New Reputation");
                    System.out.println("A.rep="+field.get(a));
                }
                if(field.getName().equalsIgnoreCase("count")){
                    field.set(a,25);
                    System.out.println("A.count="+field.get(a));
                }
            }                
        }catch(Exception err){
            err.printStackTrace();
        }
    }
}

class A {
    private String name;
    public int age;
    public final String rep;
    public static int count=0;
    
    public A(){
        name = "Unset";
        age = 0;
        rep = "Reputation";
        count++;
    }
}

Output:

A.name=StackOverFlow
A.age=20
A.rep=New Reputation
A.count=25

Explanation:

In normal scenario, private variables can't be accessed outside of declared class ( without getter and setter methods). final variables can't be re-assigned after initialization.

Reflection breaks both barriers can be abused to change both private and final variables as explained above.

field.setAccessible(true) is the key to achieve desired functionality.
  Evil Java hacks with Reflection

The Reflection API could be used to change values of private and final fields even in the JDK default library. This could be used to manipulate the behaviour of some well known classes as we will see.

What is not possible

Lets start first with the only limitation means the only field we can't change with Reflection. That is the Java SecurityManager. It is declared in java.lang.System (opens new window) as

private static volatile SecurityManager security = null;

But it won't be listed in the System class if we run this code

for(Field f : System.class.getDeclaredFields())
    System.out.println(f);

Thats because of the fieldFilterMap in sun.reflect.Reflection (opens new window) that holds the map itself and the security field in the System.class and protects them against any access with Reflection. So we could not deactivate the SecurityManager.

Crazy Strings

Each Java String is represented by the JVM as an instance of the String class. However, in some situations the JVM saves heap space by using the same instance for Strings that are. This happens for string literals, and also for strings that have been "interned" by calling String.intern(). So if you have "hello" in your code multiple times it is always the same object instance.

Strings are supposed to be immutable, but it is possible to use "evil" reflection to change them. The example below show how we can change the characters in a String by replacing its value field.

public class CrazyStrings {
    static {
        try {
            Field f = String.class.getDeclaredField("value");
            f.setAccessible(true);
            f.set("hello", "you stink!".toCharArray());
        } catch (Exception e) {
        }
    }
    public static void main(String args[])  {
        System.out.println("hello");
    }
}

So this code will print "you stink!"

1 = 42

The same idea could be used with the Integer Class

public class CrazyMath {
    static {
        try {
            Field value = Integer.class.getDeclaredField("value");    
            value.setAccessible(true);          
            value.setInt(Integer.valueOf(1), 42);      
        } catch (Exception e) {
        }
    }
    public static void main(String args[])  {
        System.out.println(Integer.valueOf(1));
    }
}

Everything is true

And according to this stackoverflow post (opens new window) we can use reflection to do something really evil.

public class Evil {    
    static {
        try {
            Field field = Boolean.class.getField("FALSE");
            field.setAccessible(true);
            Field modifiersField = Field.class.getDeclaredField("modifiers");
            modifiersField.setAccessible(true);
            modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
            field.set(null, true);
        } catch (Exception e) {
        }
    }
    public static void main(String args[]){
        System.out.format("Everything is %s", false);
    }
}

Note that what we are doing here is going to cause the JVM to behave in inexplicable ways. This is very dangerous.
  Call constructor

Getting the Constructor Object

You can obtain Constructor class from the Class object like this:

Class myClass = ... // get a class object
Constructor[] constructors = myClass.getConstructors();

Where the constructors variable will have one Constructor instance for each public constructor declared in the class.

If you know the precise parameter types of the constructor you want to access, you can filter the specific constructor. The next example returns the public constructor of the given class which takes a Integer as parameter:

Class myClass = ... // get a class object
Constructor constructor = myClass.getConstructor(new Class[]{Integer.class});

If no constructor matches the given constructor arguments a NoSuchMethodException is thrown.

New Instance using Constructor Object

Class myClass = MyObj.class // get a class object
Constructor constructor = myClass.getConstructor(Integer.class);
MyObj myObj = (MyObj) constructor.newInstance(Integer.valueOf(123));
  Invoking a method

Using reflection, a method of an object can be invoked during runtime.

The example shows how to invoke the methods of a String object.

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

String s = "Hello World!";

// method without parameters
// invoke s.length()
Method method1 = String.class.getMethod("length");
int length = (int) method1.invoke(s); // variable length contains "12"

// method with parameters
// invoke s.substring(6)
Method method2 = String.class.getMethod("substring", int.class);
String substring = (String) method2.invoke(s, 6); // variable substring contains "World!"
  Getting the Constants of an Enumeration

Giving this enumeration as Example:

enum Compass {
    NORTH(0),
    EAST(90),
    SOUTH(180),
    WEST(270);
    private int degree;
    Compass(int deg){
        degree = deg;
    }
    public int getDegree(){
        return degree;
    }
}

In Java an enum class is like any other class but has some definied constants for the enum values. Additionally it has a field that is an array that holds all the values and two static methods with name values() and valueOf(String).
We can see this if we use Reflection to print all fields in this class

for(Field f : Compass.class.getDeclaredFields())
    System.out.println(f.getName());

the output will be:

NORTH
EAST
SOUTH
WEST
degree
ENUM$VALUES

So we could examine enum classes with Reflection like any other class. But the Reflection API offers three enum-specific methods.

enum check

Compass.class.isEnum();

Returns true for classes that represents an enum type.

retrieving values

Object[] values = Compass.class.getEnumConstants();

Returns an array of all enum values like Compass.values() but without the need of an instance.

enum constant check

for(Field f : Compass.class.getDeclaredFields()){
    if(f.isEnumConstant())
        System.out.println(f.getName());
}

Lists all the class fields that are enum values.
  Get Class given its (fully qualified) name

Given a String containing the name of a class, it's Class object can be accessed using Class.forName:

Class clazz = null;
try {
    clazz = Class.forName("java.lang.Integer");
} catch (ClassNotFoundException ex) {
    throw new IllegalStateException(ex);
}

It can be specified, if the class should be initialized (second parameter of forName) and which ClassLoader should be used (third parameter):

ClassLoader classLoader = ...
boolean initialize = ...
Class clazz = null;
try {
    clazz = Class.forName("java.lang.Integer", initialize, classLoader);
} catch (ClassNotFoundException ex) {
    throw new IllegalStateException(ex);
}
  Call overloaded constructors using reflection

Example: Invoke different constructors by passing relevant parameters

import java.lang.reflect.*;

class NewInstanceWithReflection{
    public NewInstanceWithReflection(){
        System.out.println("Default constructor");
    }
    public NewInstanceWithReflection( String a){
        System.out.println("Constructor :String => "+a);
    }
    public static void main(String args[]) throws Exception {
        
        NewInstanceWithReflection object = (NewInstanceWithReflection)Class.forName("NewInstanceWithReflection").newInstance();
        Constructor constructor = NewInstanceWithReflection.class.getDeclaredConstructor( new Class[] {String.class});
        NewInstanceWithReflection object1 = (NewInstanceWithReflection)constructor.newInstance(new Object[]{"StackOverFlow"});
        
    }
}

output:

Default constructor
Constructor :String => StackOverFlow

Explanation:

  1. Create instance of class using Class.forName : It calls default constructor
  2. Invoke getDeclaredConstructor of the class by passing type of parameters as Class array
  3. After getting the constructor, create newInstance by passing parameter value as Object array
  Call constructor of nested class

If you want to create an instance of an inner nested class you need to provide a class object of the enclosing class as an extra parameter with Class#getDeclaredConstructor (opens new window).

public class Enclosing{
    public class Nested{
    public Nested(String a){
            System.out.println("Constructor :String => "+a);
        }
    }       
    public static void main(String args[]) throws Exception {
        Class<?> clazzEnclosing = Class.forName("Enclosing");            
        Class<?> clazzNested = Class.forName("Enclosing$Nested");
        Enclosing objEnclosing = (Enclosing)clazzEnclosing.newInstance();
        Constructor<?> constructor = clazzNested.getDeclaredConstructor(new Class[]{Enclosing.class, String.class});
        Nested objInner = (Nested)constructor.newInstance(new Object[]{objEnclosing, "StackOverFlow"});
    }
}

If the nested class is static you will not need this enclosing instance.

Remarks

Performance

Keep in mind that reflection might decrease performance, only use it when your task cannot be completed without reflection.

From the Java tutorial The Reflection API (opens new window) :

Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications.

 

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