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

In order to access private fields, you need to get them from the class's declared fields and then make them accessible:

Field f = obj.getClass().getDeclaredField("stuffIWant"); //NoSuchFieldException
f.setAccessible(true);
Hashtable iWantThis = (Hashtable) f.get(obj); //IllegalAccessException

EDIT: as has been commented by aperkins, both accessing the field, setting it as accessible and retrieving the value can throw Exceptions, although the only checked exceptions you need to be mindful of are commented above.

The NoSuchFieldException would be thrown if you asked for a field by a name which did not correspond to a declared field.

obj.getClass().getDeclaredField("misspelled"); //will throw NoSuchFieldException

The IllegalAccessException would be thrown if the field was not accessible (for example, if it is private and has not been made accessible via missing out the f.setAccessible(true) line.

The RuntimeExceptions which may be thrown are either SecurityExceptions (if the JVM's SecurityManager will not allow you to change a field's accessibility), or IllegalArgumentExceptions, if you try and access the field on an object not of the field's class's type:

f.get("BOB"); //will throw IllegalArgumentException, as String is of the wrong type

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.

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()

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?

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.

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");

Java 9 introduced Variable Handles. You can access a private field of a class using them.

The code for your example will look like following:

var lookup = MethodHandles.lookup();
var handle = MethodHandles
    .privateLookupIn(IWasDesignedPoorly.class, lookup)
    .findVarHandle(IWasDesignedPoorly.class, "stuffIWant", Hashtable.class);
var value = handle.get(obj);

It is also advisable to use Lookup and VarHandle objects as static final fields.

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");
    }
}

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.

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.