Here are a few common problems encountered by developers with explanations for why the occur and how to resolve them.

IllegalArgumentException due to Inconvertible Types

The FieldTrouble example will generate an IllegalArgumentException. Field.setInt() is invoked to set a field that is of the reference type Integer with a value of primitive type. In the non-reflection equivalent Integer val = 42, the compiler would convert (or box) the primitive type 42 to a reference type as new Integer(42) so that its type checking will accept the statement. When using reflection, type checking only occurs at runtime so there is no opportunity to box the value.

import java.lang.reflect.Field;

public class FieldTrouble {
    public Integer val;

    public static void main(String... args) {
	FieldTrouble ft = new FieldTrouble();
	try {
	    Class<?> c = ft.getClass();
	    Field f = c.getDeclaredField("val");
  	    f.setInt(ft, 42);               // IllegalArgumentException

        // production code should handle these exceptions more gracefully
	} catch (NoSuchFieldException x) {
	    x.printStackTrace();
 	} catch (IllegalAccessException x) {
 	    x.printStackTrace();
	}
    }
}

$ java FieldTrouble
Exception in thread "main" java.lang.IllegalArgumentException: Can not set
java.lang.Object field FieldTrouble.val to (long)42
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:146)
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:174)
        at sun.reflect.UnsafeObjectFieldAccessorImpl.setLong(UnsafeObjectFieldAccessorImpl.java:102)
        at java.lang.reflect.Field.setLong(Field.java:831)
        at FieldTrouble.main(FieldTrouble.java:11)

To eliminate this exception, the problematic line should be replaced by the following invocation of Field.set(Object obj, Object value):

f.set(ft, new Integer(43));

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Tip: When using reflection to set or get a field, the compiler does not have an opportunity to perform boxing. It can only convert types that are related as described by the specification for Class.isAssignableFrom(). The example is expected to fail because isAssignableFrom() will return false in this test which can be used programmatically to verify whether a particular conversion is possible:

Integer.class.isAssignableFrom(int.class) == false

Similarly, automatic conversion from primitive to reference type is also impossible in reflection.

int.class.isAssignableFrom(Integer.class) == false

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NoSuchFieldException for Non-Public Fields

The astute reader may notice that if the FieldSpy example shown earlier is used to get information on a non-public field, it will fail:

$ java FieldSpy java.lang.String count
java.lang.NoSuchFieldException: count
        at java.lang.Class.getField(Class.java:1519)
        at FieldSpy.main(FieldSpy.java:12)

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Tip: The Class.getField() and Class.getFields() methods return the public member field(s) of the class, enum, or interface represented by the Class object. To retrieve all fields declared (but not inherited) in the Class, use the Class.getDeclaredFields() method.

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IllegalAccessException when Modifying Final Fields

An IllegalAccessException may be thrown if an attempt is made to get or set the value of a private or otherwise inaccessible field or to set the value of a final field (regardless of its access modifiers).

The FieldTroubleToo example illustrates the type of stack trace which results from attempting to set a final field.

import java.lang.reflect.Field;

public class FieldTroubleToo {
    public final boolean b = true;

    public static void main(String... args) {
	FieldTroubleToo ft = new FieldTroubleToo();
	try {
	    Class<?> c = ft.getClass();
	    Field f = c.getDeclaredField("b");
// 	    f.setAccessible(true);  // solution
	    f.setBoolean(ft, Boolean.FALSE);   // IllegalAccessException

        // production code should handle these exceptions more gracefully
	} catch (NoSuchFieldException x) {
	    x.printStackTrace();
	} catch (IllegalArgumentException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}

$ java FieldTroubleToo
java.lang.IllegalAccessException: Can not set final boolean field FieldTroubleToo.b to (boolean)false
        at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:55)
        at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:63)
        at sun.reflect.UnsafeQualifiedBooleanFieldAccessorImpl.setBoolean(UnsafeQualifiedBooleanFieldAccessorImpl.java:78)
        at java.lang.reflect.Field.setBoolean(Field.java:686)
        at FieldTroubleToo.main(FieldTroubleToo.java:12)

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Tip: An access restriction exists which prevents final fields from being set after initialization of the class. However, Field is declared to extend AccessibleObject which provides the ability to suppress this check.

If AccessibleObject.setAccessible() succeeds, then subsequent operations on this field value will not fail do to this problem. This may have unexpected side-effects; for example, sometimes the original value will continue to be used by some sections of the application even though the value has been modified. AccessibleObject.setAccessible() will only succeed if the operation is allowed by the security context.

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