IllegalAccessException when attempting to use newInstance()

[Richard Hayward]

Could anyone tell me why one of these examples works and the other doesn't?

My first example compiles but throws an exception.

class NewInstanceTest1{
  private static class Dog{
    void bark(){System.out.println("woof!");}
  }
  public static void main(String[] args){
    Dog dog = new Dog();
    dog.bark();                           //woof!
    Class dogClass = dog.getClass();
    try{
      Object o = dogClass.newInstance();  //IllegalAccessException
    } catch(Exception e) {
      System.out.println(e);
    }
  }
}

According to the api for Class
'IllegalAccessException is thrown  if the class or its nullary constructor is not accessible.'

I'm thinking that the fact that the dog barks at line 7 demonstrates that neither of those things is true.

My second example, (simplified from the example at TutorialsPoint ) compiles and runs ok.

import java.util.Date;

public class NewInstanceTest2 {
  public static void main(String[] args) {
    Date date = new Date();
    System.out.println(date);                //ok
    Class dateClass = date.getClass();
    try {
      Object o = dateClass.newInstance();
      System.out.println(o);                 //ok
    } catch(Exception e) {
      System.out.println(e);
    }  
  }
}

Seems to me however that the second example is doing the same thing.

[Campbell Ritchie]

That tutorial is horribly out of date if it recommends using java.util.Date. A dreadful class, which was largely superseded by Calendar nearly twenty years ago, and should have been completely superseded by newer classes about five years ago. It also doesn't tell you that you oughtn't to use newInstance() any more.
Anyway, Date is a public class in its package and it has a public constructor without any arguments, so it is accessible to newInstance(). So far, so good. So  tried your code and didn't get any exceptions.

[Richard Hayward]

Thanks for the tips about Date. It's not primarily Date I was interested in, any class would do. I just came across that code snippet when googling for an example of the use of newInstance().

Campbell Ritchie wrote: It also doesn't tell you that you oughtn't to use newInstance() any more.

ok. That link for the java 12 api shows newInstance as deprecated since java 9. I'm using a java 8 compiler.

Campbell Ritchie wrote:
So  tried your code and didn't get any exceptions.

Should still work though shouldn't it?

richard@ProBook-450-G2:~/projects/test$ javac -version
javac 1.8.0_211
richard@ProBook-450-G2:~/projects/test$ javac NewInstanceTest1.java 
richard@ProBook-450-G2:~/projects/test$ java NewInstanceTest1 
woof!
java.lang.IllegalAccessException: Class NewInstanceTest1 can not access a member of class NewInstanceTest1$Dog with modifiers "private"
richard@ProBook-450-G2:~/projects/test$ 

[Stephan van Hulst]

The error message tells you exactly why it can't create an instance of NewInstanceTest1$Dog: It has the private access modifier.

[Campbell Ritchie]

So why could I get Java12 to create an instance without any exceptions last night? I shall try again.

Campbell's Java12/Linux/x64 wrote:[critchie@...~]$ pluma NewInstanceTest2.java&
[1] 31972
[critchie@... ~]$ javac NewInstanceTest2.java
Note: NewInstanceTest2.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
[critchie@... ~]$ java NewInstanceTest2
Fri Apr 26 10:12:58 BST 2019
Fri Apr 26 10:12:58 BST 2019
[critchie@...~]$ pluma NewInstanceTest1.java&
[2] 32147
[critchie@... ~]$ javac NewInstanceTest1.java
Note: NewInstanceTest1.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
[2]+  Done                    pluma NewInstanceTest1.java
[critchie@... ~]$ java NewInstanceTest1
woof!
[critchie@... ~]$  

Code used copied'n'pasted from above post unchanged. Insert “System.out.println(o);” after line 10 in first class, and I get this instead:-

. . . java NewInstanceTest1
woof!
NewInstanceTest1$Dog@4d7e1886

Note typical output from un‑overridden toString() method.

[Dave Tolls]

Maybe the code has changed?

[Stephan van Hulst]

Wow, apparently this was fixed by a feature implementation in Java 11: http://openjdk.java.net/jeps/181

I'm guessing that direct access to private members of nested/enclosing classes was implemented through bridge-methods before Java 11. Bridge methods are added by the compiler, so they don't account for reflective access.

Java 11 added special access scopes that allow access to these private members without using bridge methods, which would also work when using reflection.

[edit]

From the JEP I linked to above:

JEP 181 wrote:
A further consequence of the lack of JVM support for private access within a nest, is that core reflection also denies access.
A reflective method invocation (using java.lang.reflect.Method.invoke) from one nestmate to another throws IllegalAccessError (unless access control has been disabled).
This is surprising given that reflective invocations should behave the same as source level invocations.

[Richard Hayward]

So could I clarify this?

Stephan van Hulst wrote:The error message tells you exactly why it can't create an instance of NewInstanceTest1$Dog: It has the private access modifier.

I didn't understand Stephan's response here because I thought that as far as inner classes are concerned, the outer class can see members of an inner class, even if they are declared private. For example:

class PrivateInnerAccessTest{
  private static class Dog{
    private void bark(){System.out.println("woof!");}
  }
  public static void main(String[] args){
    Dog dog = new Dog();
    dog.bark();                           //woof!
  }
}

the dog barks. Therefore, from my initial post

class NewInstanceTest1{
  private static class Dog{
    void bark(){System.out.println("woof!");}
  }
  public static void main(String[] args){
    Dog dog = new Dog();
    dog.bark();                           //woof!
    Class dogClass = dog.getClass();
    try{
      Object o = dogClass.newInstance();  //IllegalAccessException
    } catch(Exception e) {
      System.out.println(e);
    }
  }
}

I was thinking that because Dog is an inner class, the outer class should be able access all it methods. Is the problem that it's Dog that's the inner class, not dogClass?

[Rob Spoor]

Reflection in general is more limited than compiled code. While you can easily call a private method directly, if you retrieve that method through reflection you must first make it accessible (setAccessible(true)).

[Richard Hayward]

Thanks to all for your help on this thread.

Seems I should spend a bit of time exploring what levels of access are given to objects when reflection is used, as opposed to when it's not.

[Campbell Ritchie]

Maybe it would be better simply to regard reflection as problematic and use it as little as possible.