Spec-Zone .ru
спецификации, руководства, описания, API
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The following examples show typical errors which may be encountered when reflecting on classes.
When a method is invoked, the types of the argument values are checked and possibly converted.
invokes
ClassWarning
getMethod()
to cause a typical unchecked conversion warning:
import java.lang.reflect.Method; public class ClassWarning { void m() { try { Class c = ClassWarning.class; Method m = c.getMethod("m"); // warning // production code should handle this exception more gracefully } catch (NoSuchMethodException x) { x.printStackTrace(); } } }
$ javac ClassWarning.java Note: ClassWarning.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. $ javac -Xlint:unchecked ClassWarning.java ClassWarning.java:6: warning: [unchecked] unchecked call to getMethod (String,Class<?>...) as a member of the raw type Class Method m = c.getMethod("m"); // warning ^ 1 warning
Many library methods have been retrofitted with generic declarations including several in
Class
c
is declared as a raw type (has no type parameters) and the corresponding parameter of
getMethod()
There are two possible solutions. The more preferable it to modify the declaration of c
to include an appropriate generic type. In this case, the declaration should be:
Class<?> c = warn.getClass();
Alternatively, the warning could be explicitly suppressed using the predefined annotation
@SuppressWarnings
Class c = ClassWarning.class; @SuppressWarnings("unchecked") Method m = c.getMethod("m"); // warning gone
@SuppressWarnings
Class.newInstance()
InstantiationException
example illustrates the resulting stack trace.ClassTrouble
class Cls { private Cls() {} } public class ClassTrouble { public static void main(String... args) { try { Class<?> c = Class.forName("Cls"); c.newInstance(); // InstantiationException // production code should handle these exceptions more gracefully } catch (InstantiationException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } catch (ClassNotFoundException x) { x.printStackTrace(); } } }
$ java ClassTrouble java.lang.IllegalAccessException: Class ClassTrouble can not access a member of class Cls with modifiers "private" at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65) at java.lang.Class.newInstance0(Class.java:349) at java.lang.Class.newInstance(Class.java:308) at ClassTrouble.main(ClassTrouble.java:9)
Class.newInstance()
behaves very much like the new
keyword and will fail for the same reasons new
would fail. The typical solution in reflection is to take advantage of the
java.lang.reflect.AccessibleObject
java.lang.Class
AccessibleObject
Constructor.newInstance()
AccessibleObject
Constructor.newInstance()
Additional examples of potential problems using
Constructor.newInstance()