[java] Any way to Invoke a private method?

I have a class that uses XML and reflection to return Objects to another class.

Normally these objects are sub fields of an external object, but occasionally it's something I want to generate on the fly. I've tried something like this but to no avail. I believe that's because Java won't allow you to access private methods for reflection.

Element node = outerNode.item(0);
String methodName = node.getAttribute("method");
String objectName = node.getAttribute("object");

if ("SomeObject".equals(objectName))
    object = someObject;
else
    object = this;

method = object.getClass().getMethod(methodName, (Class[]) null);

If the method provided is private, it fails with a NoSuchMethodException. I could solve it by making the method public, or making another class to derive it from.

Long story short, I was just wondering if there was a way to access a private method via reflection.

you can do this using ReflectionTestUtils of Spring (org.springframework.test.util.ReflectionTestUtils)

ReflectionTestUtils.invokeMethod(instantiatedObject,"methodName",argument);

Example : if you have a class with a private method square(int x)

Calculator calculator = new Calculator();
ReflectionTestUtils.invokeMethod(calculator,"square",10);

If the method accepts non-primitive data type then the following method can be used to invoke a private method of any class:

public static Object genericInvokeMethod(Object obj, String methodName,
            Object... params) {
        int paramCount = params.length;
        Method method;
        Object requiredObj = null;
        Class<?>[] classArray = new Class<?>[paramCount];
        for (int i = 0; i < paramCount; i++) {
            classArray[i] = params[i].getClass();
        }
        try {
            method = obj.getClass().getDeclaredMethod(methodName, classArray);
            method.setAccessible(true);
            requiredObj = method.invoke(obj, params);
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }

        return requiredObj;
    }

The Parameter accepted are obj, methodName and the parameters. For example

public class Test {
private String concatString(String a, String b) {
    return (a+b);
}
}

Method concatString can be invoked as

Test t = new Test();
    String str = (String) genericInvokeMethod(t, "concatString", "Hello", "Mr.x");

You can use Manifold's @Jailbreak for direct, type-safe Java reflection:

@Jailbreak Foo foo = new Foo();
foo.callMe();

public class Foo {
    private void callMe();
}

@Jailbreak unlocks the foo local variable in the compiler for direct access to all the members in Foo's hierarchy.

Similarly you can use the jailbreak() extension method for one-off use:

foo.jailbreak().callMe();

Through the jailbreak() method you can access any member in Foo's hierarchy.

In both cases the compiler resolves the method call for you type-safely, as if a public method, while Manifold generates efficient reflection code for you under the hood.

Alternatively, if the type is not known statically, you can use Structural Typing to define an interface a type can satisfy without having to declare its implementation. This strategy maintains type-safety and avoids performance and identity issues associated with reflection and proxy code.

Discover more about Manifold.

One more variant is using very powerfull JOOR library https://github.com/jOOQ/jOOR

MyObject myObject = new MyObject()
on(myObject).get("privateField");  

It allows to modify any fields like final static constants and call yne protected methods without specifying concrete class in the inheritance hierarhy

<!-- https://mvnrepository.com/artifact/org.jooq/joor-java-8 -->
<dependency>
     <groupId>org.jooq</groupId>
     <artifactId>joor-java-8</artifactId>
     <version>0.9.7</version>
</dependency>

Let me provide complete code for execution protected methods via reflection. It supports any types of params including generics, autoboxed params and null values

@SuppressWarnings("unchecked")
public static <T> T executeSuperMethod(Object instance, String methodName, Object... params) throws Exception {
    return executeMethod(instance.getClass().getSuperclass(), instance, methodName, params);
}

public static <T> T executeMethod(Object instance, String methodName, Object... params) throws Exception {
    return executeMethod(instance.getClass(), instance, methodName, params);
}

@SuppressWarnings("unchecked")
public static <T> T executeMethod(Class clazz, Object instance, String methodName, Object... params) throws Exception {

    Method[] allMethods = clazz.getDeclaredMethods();

    if (allMethods != null && allMethods.length > 0) {

        Class[] paramClasses = Arrays.stream(params).map(p -> p != null ? p.getClass() : null).toArray(Class[]::new);

        for (Method method : allMethods) {
            String currentMethodName = method.getName();
            if (!currentMethodName.equals(methodName)) {
                continue;
            }
            Type[] pTypes = method.getParameterTypes();
            if (pTypes.length == paramClasses.length) {
                boolean goodMethod = true;
                int i = 0;
                for (Type pType : pTypes) {
                    if (!ClassUtils.isAssignable(paramClasses[i++], (Class<?>) pType)) {
                        goodMethod = false;
                        break;
                    }
                }
                if (goodMethod) {
                    method.setAccessible(true);
                    return (T) method.invoke(instance, params);
                }
            }
        }

        throw new MethodNotFoundException("There are no methods found with name " + methodName + " and params " +
            Arrays.toString(paramClasses));
    }

    throw new MethodNotFoundException("There are no methods found with name " + methodName);
}

Method uses apache ClassUtils for checking compatibility of autoboxed params

Use getDeclaredMethod() to get a private Method object and then use method.setAccessible() to allow to actually call it.