IllegalAccessException on using reflection

Question

I was trying to learn reflection and I came across this IllegalAccessException. Please see the following code:

public class ReflectionTest
{
      public static void main(String[] args)
      {
           Set<String> myStr = new HashSet<String>();
           myStr.add("obj1");
           Iterator itr = myStr.iterator();
           Method mtd = itr.getClass().getMethod("hasNext");
           System.out.println(m.invoke(it));
      }
}

When I tried to run this program, I got the following:

Exception in thread "main" IllegalAccessException

I don't understand what's going on. Any ideas? Thanks in advance.

Should `System.out.println(m.invoke(it));` be `System.out.println(mtd.invoke(itr));`? Looks like you're trying to access a private member. — slau, Mar 03 '11 at 17:38

score 8 · Answer 1 · answered Mar 03 '11 at 17:48

8

The troublesome piece of code is this:

itr.getClass().getMethod

You probably wanted hasNext on the Iterator class. What you have written is the HashMap.KeyIterator class, which according the Java language access specifiers (or at least the rough interpretation of JDK 1.0 used by reflection) is not available to your code.

Use instead:

Iterator.class.getMethod

(And if it wasn't for learning purposes, stay away from reflection.)

answered Mar 03 '11 at 17:48

Tom Hawtin - tackline

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Why discourage the use of reflection? It can be a very powerful and helpful tool: https://softwareengineering.stackexchange.com/questions/101187/are-there-problems-with-using-reflection – delucasvb May 08 '17 at 13:38
Please don't discourage a useful tool with no elaboration nor justification. – naomimyselfandi May 13 '18 at 15:06

score 2 · Answer 2 · edited May 23 '17 at 12:17

2

You cannot access it, because the Iterator is a private inner class. More explanation can be found here.

edited May 23 '17 at 12:17

Community

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1

answered Mar 03 '11 at 17:48

Zsolt

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Johan Sjöberg · Answer 3 · 2011-03-03T18:16:55.133

0

It's apparent that your currently executing method does not have access to the method named hasNext, e.g., by it being private or protected. You could try to enable access to it using method.setAccessible(true);

It might also be that you have some restrictions defined in your security manager (which, if you use e.g., linux, might have been included default from the distributions java package).

[EDIT] As it turns out, Tom Hawtin identified the correct root cause. You are indeed operating on HashMap.KeyIterator. Although the solution would be to use Iterator.class instead of itr.getClass() you could still enable access to it using setAccessible(true).

edited Mar 03 '11 at 18:16

answered Mar 03 '11 at 17:37

Johan Sjöberg

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`hasNext` isn't private. Your second statement may be true. – adarshr Mar 03 '11 at 17:42
@adarshr, as it turns out `hasNext` *[has private](http://www.java2s.com/Open-Source/Java-Document/6.0-JDK-Core/Collections-Jar-Zip-Logging-regex/java/util/HashMap.java.htm)* access on the iterator of type `HashMap.KeyIterator` which your code operatons on. – Johan Sjöberg Mar 03 '11 at 18:20

score 0 · Answer 4 · answered Mar 03 '11 at 22:40

I suspect you should use getDeclaredMethod (among other issues). I don't bother to remember Reflection API details (they're for the compiler!), but in your case compare your code with that produced by dp4j:

$ javac -Averbose=true -All -cp dp4j-1.2-SNAPSHOT-jar-with-dependencies.jar ReflectionTest.java 
ReflectionTest.java:6: Note: 
import java.util.*;

public class ReflectionTest {

public ReflectionTest() {
    super();
}

@com.dp4j.Reflect()
public static void main(String[] args) throws java.lang.ClassNotFoundException, java.lang.NoSuchFieldException, java.lang.IllegalAccessException, java.lang.NoSuchMethodException, java.lang.reflect.InvocationTargetException, java.lang.InstantiationException, java.lang.IllegalArgumentException {
    final java.lang.reflect.Constructor hashSetConstructor = Class.forName("java.util.HashSet").getDeclaredConstructor();
    hashSetConstructor.setAccessible(true);
    Set<String> myStr = (.java.util.Set<.java.lang.String>)hashSetConstructor.newInstance();
    final java.lang.reflect.Method addWithEMethod = Class.forName("java.util.Set").getDeclaredMethod("add", .java.lang.Object.class);
    addWithEMethod.setAccessible(true);
    addWithEMethod.invoke(myStr, new .java.lang.Object[1][]{"obj1"});
    final java.lang.reflect.Method iteratorMethod = Class.forName("java.util.Set").getDeclaredMethod("iterator");
    iteratorMethod.setAccessible(true);
    Iterator itr = (.java.util.Iterator)iteratorMethod.invoke(myStr);
    final java.lang.reflect.Method hasNextMethod = Class.forName("java.util.Iterator").getDeclaredMethod("hasNext");
    hasNextMethod.setAccessible(true);
    final java.lang.reflect.Method printlnWithbooleanMethod = Class.forName("java.io.PrintStream").getDeclaredMethod("println", .java.lang.Boolean.TYPE);
    printlnWithbooleanMethod.setAccessible(true);
    printlnWithbooleanMethod.invoke(System.out, new .java.lang.Object[1][]{hasNextMethod.invoke(itr)});
}

}

    public static void main(String[] args)
                       ^
...

$ java ReflectionTest
true

The only change you need to do is annotate your main method with @com.dp4j.Reflect:

$ vim ReflectionTest.java
import java.util.*;

public class ReflectionTest
{
        @com.dp4j.Reflect
        public static void main(String[] args)
        {
                Set<String> myStr = new HashSet<String>();
                myStr.add("obj1");
                Iterator itr = myStr.iterator();
                // Method mtd = itr.getClass().getMethod("hasNext");
                System.out.println(itr.hasNext());
        }
}

NB: this works only with dp4j-1.2-SNAPSHOT (I've just added suport for it). If you don't use Maven, download the jar from here. You find the test case with your problem here.

IllegalAccessException on using reflection

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