[java] How to read the value of a private field from a different class in Java?

I have a poorly designed class in a 3rd-party JAR and I need to access one of its private fields. For example, why should I need to choose private field is it necessary?

class IWasDesignedPoorly {
    private Hashtable stuffIWant;
}

IWasDesignedPoorly obj = ...;

How can I use reflection to get the value of stuffIWant?

Using the Reflection in Java you can access all the private/public fields and methods of one class to another .But as per the Oracle documentation in the section drawbacks they recommended that :

"Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform"

here is following code snapts to demonstrate basic concepts of Reflection

Reflection1.java

public class Reflection1{

    private int i = 10;

    public void methoda()
    {

        System.out.println("method1");
    }
    public void methodb()
    {

        System.out.println("method2");
    }
    public void methodc()
    {

        System.out.println("method3");
    }

}

Reflection2.java

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


public class Reflection2{

    public static void main(String ar[]) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException
    {
        Method[] mthd = Reflection1.class.getMethods(); // for axis the methods 

        Field[] fld = Reflection1.class.getDeclaredFields();  // for axis the fields  

        // Loop for get all the methods in class
        for(Method mthd1:mthd)
        {

            System.out.println("method :"+mthd1.getName());
            System.out.println("parametes :"+mthd1.getReturnType());
        }

        // Loop for get all the Field in class
        for(Field fld1:fld)
        {
            fld1.setAccessible(true);
            System.out.println("field :"+fld1.getName());
            System.out.println("type :"+fld1.getType());
            System.out.println("value :"+fld1.getInt(new Reflaction1()));
        }
    }

}

Hope it will help.

Try to go around the problem for the case, the calass of which you want to set/get data is one of your own classes.

Just create a public setter(Field f, Object value) and public Object getter(Field f) for that. You can even do some securoty check on your own inside theses member functions. E.g. for the setter:

class myClassName {
    private String aString;

    public set(Field field, Object value) {
        // (A) do some checkings here  for security

        // (B) set the value
        field.set(this, value);
    }
}

Of course, now you have to find out the java.lang.reflect.Field for sString prior to setting of field value.

I do use this technique in a generic ResultSet-to-and-from-model-mapper.

As oxbow_lakes mentions, you can use reflection to get around the access restrictions (assuming your SecurityManager will let you).

That said, if this class is so badly designed that it makes you resort to such hackery, maybe you should look for an alternative. Sure this little hack might be saving you a few hours now, but how much will it cost you down the road?

Try FieldUtils from apache commons-lang3:

FieldUtils.readField(object, fieldName, true);

Use the Soot Java Optimization framework to directly modify the bytecode. http://www.sable.mcgill.ca/soot/

Soot is completely written in Java and works with new Java versions.

Reflection isn't the only way to resolve your issue (which is to access the private functionality/behaviour of a class/component)

An alternative solution is to extract the class from the .jar, decompile it using (say) Jode or Jad, change the field (or add an accessor), and recompile it against the original .jar. Then put the new .class ahead of the .jar in the classpath, or reinsert it in the .jar. (the jar utility allows you to extract and reinsert to an existing .jar)

As noted below, this resolves the wider issue of accessing/changing private state rather than simply accessing/changing a field.

This requires the .jar not to be signed, of course.

If using Spring:

In a testing context, ReflectionTestUtils provides some handy tools that can help out here with minimal effort. It's described as being "for use in unit and integration testing scenarios".

In a non-testing context, there is also a similar class named ReflectionUtils but this is described as "Only intended for internal use" - see this answer for a good interpretation of what this means.

To address the example in the original post:

Hashtable iWantThis = (Hashtable)ReflectionTestUtils.getField(obj, "stuffIWant");

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

@JailBreak Foo foo = new Foo();
foo.stuffIWant = "123;

public class Foo {
    private String stuffIWant;
}

@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().stuffIWant = "123";

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

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

Discover more about Manifold.

One other option that hasn't been mentioned yet: use Groovy. Groovy allows you to access private instance variables as a side effect of the design of the language. Whether or not you have a getter for the field, you can just use

def obj = new IWasDesignedPoorly()
def hashTable = obj.getStuffIWant()

You need to do the following:

private static Field getField(Class<?> cls, String fieldName) {
    for (Class<?> c = cls; c != null; c = c.getSuperclass()) {
        try {
            final Field field = c.getDeclaredField(fieldName);
            field.setAccessible(true);
            return field;
        } catch (final NoSuchFieldException e) {
            // Try parent
        } catch (Exception e) {
            throw new IllegalArgumentException(
                    "Cannot access field " + cls.getName() + "." + fieldName, e);
        }
    }
    throw new IllegalArgumentException(
            "Cannot find field " + cls.getName() + "." + fieldName);
}

You can use jOOR for that.

class Foo {
    private final String value = "ABC";
}
class Bar {
    private final Foo foo = new Foo();
    public String value() {
        return org.joor.Reflect
            .on(this.foo)
            .field("value")
            .get();
    }
}
class BarTest {
    @Test
    void accessPrivateField() {
        Assertions.assertEquals(new Bar().value(), "ABC");
    }
}

Just an additional note about reflection: I have observed in some special cases, when several classes with the same name exist in different packages, that reflection as used in the top answer may fail to pick the correct class from the object. So if you know what is the package.class of the object, then it's better to access its private field values as follows:

org.deeplearning4j.nn.layers.BaseOutputLayer ll = (org.deeplearning4j.nn.layers.BaseOutputLayer) model.getLayer(0);
Field f = Class.forName("org.deeplearning4j.nn.layers.BaseOutputLayer").getDeclaredField("solver");
f.setAccessible(true);
Solver s = (Solver) f.get(ll);

(This is the example class that was not working for me)

It is quite easy with the tool XrayInterface. Just define the missing getters/setters, e.g.

interface BetterDesigned {
  Hashtable getStuffIWant(); //is mapped by convention to stuffIWant
}

and xray your poor designed project:

IWasDesignedPoorly obj = new IWasDesignedPoorly();
BetterDesigned better = ...;
System.out.println(better.getStuffIWant());

Internally this relies on reflection.