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Access Checking and accessibleobject

The Field Class

The Field class defines methods for asking the type of a field and for setting and getting the value of the field. Combined with the inherited Member methods, this allows you to find out everything about the field declaration and to manipulate the field of a specific object or class. The get generic type method returns the instance of Type that represents the field’s declared type. For a plain type, such as String or int, this returns the associated Class object string. class and int. class, respectively. For a parameterized type like List, it will return a parameterized type instance. For a type variable like T, it will return a type variable instance. The gettype legacy method returns the Class object for the type of the field. For plain types, this acts the same as getting generic type.

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If the field’s declared type is a parameterized type, then gettype will return the class object for the erasure of the parameterized type that is, the class object for the raw type. For example, for a field declared as List, gettype will return List. class. If the field’s declared type is a type variable, then gettype will return the class object for the erasure of the type variable. For example, given class Foo, for a field declared with type T, gettype would return Object. class. If Foo were declared as Foo, then gettype would return Number. class. You can ask whether a field is an enum constant using isenumconstant. You can also get and set the value of a field using the get and set methods. There is a general-purpose form of these methods that take Object arguments and return Object values and more specific forms that deal directly with primitive types. All of these methods take an argument specifying which object to operate on. For static fields, the object is ignored and can be null. The following method prints the value of a short field of an object:

Final Fields

Under normal circumstances attempting to set the value of a field declared as final will result in illegal accessexception being thrown. This is what you would expect: Final fields should never have their value changed. There are special circumstances such as during custom deserialization (see page 554)where it makes sense to change the value of a final field. You can do this via reflection only on instance fields, and only if set accessible(true) has been invoked on the Field object. Note that it is not enough that set accessible(true) would succeed, it must actually be called.

This capability is provided for highly specialized contexts and is not intended for general use mention it is only for completeness. Changing a final field can have unexpected, possibly tragic consequences unless performed in specific contexts, such as custom deserialization. Outside those contexts, changes to a final field are not guaranteed to be visible. Even in such contexts, code using this technique must be guaranteed that security will not thwart its work. Changing a final field that is a constant variable (see page 46) will never result in the change being seen, except through the use of reflection don’t do it!

The Method Class

Method implements annotated elements, and the annotations on a method can be queried as discussed in Section 16.2 on page 414. Additionally, Method provides the get parameter annotations method to provide access to the annotations applied to the method’s parameters. The get parameter annotations method returns an array of Annotation arrays, with each element of the outermost array corresponding to the parameters of the method, in the order, they are declared.

If a parameter has no annotations then a zero-length Annotation array is provided for that parameter. If the Method object represents a method that is itself an element of annotation, the get default value method returns an object representing the default value of that element. If it is not an annotation element or if there is no default value, then null is returned. The Method class also implements generic declaration and so defines the method gettypep arameters which returns an array of type variable objects. If a given Method object does not present a generic method, then an empty array is returned.

Last word

The reflection-based code has semantically equivalent safety checks, although the checks that are made by the compiler for direct invocation can be made only at run time when you use invoke. The access checking may be done in a somewhat different way you might be denied access to a method in your package by the security manager, even if you could invoke that method directly.

These are good reasons to avoid using this kind of invocation when you can. It’s reasonable to use invoke or the get set methods of Field when you are writing a debugger or other generic applications that require interpreting user input as manipulations of objects. A Method object can be used somewhat like a method pointer in other languages, but there are better tools notably interfaces, abstract classes, and nested classes to address the problems typically solved by method pointers in those languages.



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