Spec-Zone .ru
спецификации, руководства, описания, API
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Reflection provides a means for invoking methods on a class. Typically, this would only be necessary if it is not possible to cast an instance of the class to the desired type in non-reflective code. Methods are invoked with
java.lang.reflect.Method.invoke()
. The first argument is the object instance on which this particular method is to be invoked. (If the method is static
, the first argument should be null
.) Subsequent arguments are the method's parameters. If the underlying method throws an exception, it will be wrapped by an
java.lang.reflect.InvocationTargetException
InvocationTargetException.getCause()
Consider a test suite which uses reflection to invoke private test methods in a given class. The
example searches for Deet
public
methods in a class which begin with the string "test
", have a boolean return type, and a single
Locale
import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.lang.reflect.Type; import java.util.Locale; import static java.lang.System.out; import static java.lang.System.err; public class Deet<T> { private boolean testDeet(Locale l) { // getISO3Language() may throw a MissingResourceException out.format("Locale = %s, ISO Language Code = %s%n", l.getDisplayName(), l.getISO3Language()); return true; } private int testFoo(Locale l) { return 0; } private boolean testBar() { return true; } public static void main(String... args) { if (args.length != 4) { err.format("Usage: java Deet <classname> <langauge> <country> <variant>%n"); return; } try { Class<?> c = Class.forName(args[0]); Object t = c.newInstance(); Method[] allMethods = c.getDeclaredMethods(); for (Method m : allMethods) { String mname = m.getName(); if (!mname.startsWith("test") || (m.getGenericReturnType() != boolean.class)) { continue; } Type[] pType = m.getGenericParameterTypes(); if ((pType.length != 1) || Locale.class.isAssignableFrom(pType[0].getClass())) { continue; } out.format("invoking %s()%n", mname); try { m.setAccessible(true); Object o = m.invoke(t, new Locale(args[1], args[2], args[3])); out.format("%s() returned %b%n", mname, (Boolean) o); // Handle any exceptions thrown by method to be invoked. } catch (InvocationTargetException x) { Throwable cause = x.getCause(); err.format("invocation of %s failed: %s%n", mname, cause.getMessage()); } } // production code should handle these exceptions more gracefully } catch (ClassNotFoundException x) { x.printStackTrace(); } catch (InstantiationException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } } }
Deet
invokes
getDeclaredMethods()
Class.isAssignableFrom()
true
since
Locale
final
:
Locale.class == pType[0].getClass()
However,
Class.isAssignableFrom()
$ java Deet Deet ja JP JP invoking testDeet() Locale = Japanese (Japan,JP), ISO Language Code = jpn testDeet() returned true
$ java Deet Deet xx XX XX invoking testDeet() invocation of testDeet failed: Couldn't find 3-letter language code for xx
First, note that only testDeet()
meets the declaration restrictions enforced by the code. Next, when testDeet()
is passed an invalid argument it throws an unchecked
java.util.MissingResourceException
InvocationTargetException
Method.invoke()
The
example illustrates how to invoke the InvokeMain
main()
entry point in any class and pass a set of arguments determined at runtime.
import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.util.Arrays; public class InvokeMain { public static void main(String... args) { try { Class<?> c = Class.forName(args[0]); Class[] argTypes = new Class[] { String[].class }; Method main = c.getDeclaredMethod("main", argTypes); String[] mainArgs = Arrays.copyOfRange(args, 1, args.length); System.out.format("invoking %s.main()%n", c.getName()); main.invoke(null, (Object)mainArgs); // production code should handle these exceptions more gracefully } catch (ClassNotFoundException x) { x.printStackTrace(); } catch (NoSuchMethodException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } catch (InvocationTargetException x) { x.printStackTrace(); } } }
First, to find the main()
method the code searches for a class with the name "main" with a single parameter that is an array of
String
Since main()
is static
, null
is the first argument to
Method.invoke()
$ java InvokeMain Deet Deet ja JP JP invoking Deet.main() invoking testDeet() Locale = Japanese (Japan,JP), ISO Language Code = jpn testDeet() returned true