I have a class that uses XML and reflection to return Object
s 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 invoke private method with reflection. Modifying the last bit of the posted code:
Method method = object.getClass().getDeclaredMethod(methodName);
method.setAccessible(true);
Object r = method.invoke(object);
There are a couple of caveats. First, getDeclaredMethod
will only find method declared in the current Class
, not inherited from supertypes. So, traverse up the concrete class hierarchy if necessary. Second, a SecurityManager
can prevent use of the setAccessible
method. So, it may need to run as a PrivilegedAction
(using AccessController
or Subject
).
使用getDeclaredMethod()
获取私有的Method对象,然后使用method.setAccessible()
允许实际调用它。
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 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);
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
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>
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 .
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