Java/Reflection/Annotation

A better concise toString method for annotation types

   <source lang="java">

import java.util.Map; import java.util.HashMap; import java.util.Collection; import java.util.ArrayList; import java.util.List; import java.util.Set; import java.util.HashSet; import java.util.SortedSet; import java.util.TreeSet; import java.util.Queue; import java.util.LinkedList; import java.util.SortedMap; import java.util.TreeMap; import java.util.Arrays; import java.util.concurrent.ConcurrentHashMap; import java.lang.annotation.Annotation; import java.lang.reflect.Method; import java.lang.reflect.Field; import java.lang.reflect.Modifier; import java.lang.reflect.ParameterizedType; import java.lang.reflect.Type; import static java.lang.reflect.Modifier.isPublic; import java.beans.PropertyDescriptor; import java.beans.BeanInfo; import java.beans.Introspector; import java.beans.IntrospectionException; /**

* Common utilty methods that are useful when working with reflection.
*
* @author Tim Fennell
*/

public class ReflectUtil {

   /** A cache of property descriptors by class and property name */
   private static Map<Class<?>, Map<String, PropertyDescriptor>> propertyDescriptors =
           new ConcurrentHashMap<Class<?>, Map<String, PropertyDescriptor>>();
   /** Static helper class, shouldn"t be constructed. */
   private ReflectUtil() {}
   /**
    * Holds a map of commonly used interface types (mostly collections) to a class that
    * implements the interface and will, by default, be instantiated when an instance
    * of the interface is needed.
    */
   protected static final Map<Class<?>,Class<?>> interfaceImplementations = new HashMap<Class<?>,Class<?>>();
   /**
    * Holds a map of primitive type to the default value for that primitive type.  Isn"t it
    * odd that there"s no way to get this programmatically from the Class objects?
    */
   protected static final Map<Class<?>,Object> primitiveDefaults = new HashMap<Class<?>,Object>();
   static {
       interfaceImplementations.put(Collection.class, ArrayList.class);
       interfaceImplementations.put(List.class,       ArrayList.class);
       interfaceImplementations.put(Set.class,        HashSet.class);
       interfaceImplementations.put(SortedSet.class,  TreeSet.class);
       interfaceImplementations.put(Queue.class,      LinkedList.class);
       interfaceImplementations.put(Map.class,        HashMap.class);
       interfaceImplementations.put(SortedMap.class,  TreeMap.class);
       primitiveDefaults.put(Boolean.TYPE,    false);
       primitiveDefaults.put(Character.TYPE, "\0");
       primitiveDefaults.put(Byte.TYPE,       new Byte("0"));
       primitiveDefaults.put(Short.TYPE,      new Short("0"));
       primitiveDefaults.put(Integer.TYPE,    new Integer(0));
       primitiveDefaults.put(Long.TYPE,       new Long(0l));
       primitiveDefaults.put(Float.TYPE,      new Float(0f));
       primitiveDefaults.put(Double.TYPE,     new Double(0.0));
   }
   /**
    * The set of method that annotation classes inherit, and should be avoided when
    * toString()ing an annotation class.
    */
   private static final Set<String> INHERITED_ANNOTATION_METHODS =
           new HashSet(Arrays.asList("toString", "equals", "hashCode", "annotationType"));
   /**
    * Looks up the default implementing type for the supplied interface. This is done
    * based on a static map of known common interface types and implementing classes.
    *
    * @param iface an interface for which an implementing class is needed
    * @return a Class object representing the implementing type, or null if one is
    *         not found
    */
   public static Class<?> getImplementingClass(Class<?> iface) {
       return interfaceImplementations.get(iface);
   }
   /**
    * Attempts to determine an implementing class for the interface provided and instantiate
    * it using a default constructor.
    *
    * @param interfaceType an interface (or abstract class) to make an instance of
    * @return an instance of the interface type supplied
    * @throws InstantiationException if no implementation type has been configured
    * @throws IllegalAccessException if thrown by the JVM during class instantiation
    */
   @SuppressWarnings("unchecked")
 public static <T> T getInterfaceInstance(Class<T> interfaceType)
           throws InstantiationException, IllegalAccessException {
       Class impl = getImplementingClass(interfaceType);
       if (impl == null) {
           throw new InstantiationException(
                   "Stripes needed to instantiate a property who"s declared type as an " +
                   "interface (which obviously cannot be instantiated. The interface is not " +
                   "one that Stripes is aware of, so no implementing class was known. The " +
                   "interface type was: "" + interfaceType.getName() + "". To fix this " +
                   "you"ll need to do one of three things. 1) Change the getter/setter methods " +
                   "to use a concrete type so that Stripes can instantiate it. 2) in the bean"s " +
                   "setContext() method pre-instantiate the property so Stripes doesn"t have to. " +
                   "3) Bug the Stripes author ;)  If the interface is a JDK type it can easily be " +
                   "fixed. If not, if enough people ask, a generic way to handle the problem " +
                   "might get implemented.");
       }
       else {
           return (T) impl.newInstance();
       }
   }
   /**
    * Utility method used to load a class.  Any time that Stripes needs to load of find a
    * class by name it uses this method.  As a result any time the classloading strategy
    * needs to change it can be done in one place!  Currently uses
    * {@code Thread.currentThread().getContextClassLoader().loadClass(String)}.
    *
    * @param name the fully qualified (binary) name of the class to find or load
    * @return the Class object representing the class
    * @throws ClassNotFoundException if the class cannot be loaded
    */
   @SuppressWarnings("unchecked") // this allows us to assign without casting
 public static Class findClass(String name) throws ClassNotFoundException {
       return Thread.currentThread().getContextClassLoader().loadClass(name);
   }
   /**

*

A better (more concise) toString method for annotation types that yields a String * that should look more like the actual usage of the annotation in a class. The String produced * is similar to that produced by calling toString() on the annotation directly, with the * following differences:

    *

*

*
• Uses the classes simple name instead of it"s fully qualified name.
*
• Only outputs attributes that are set to non-default values.

    *

*

If, for some unforseen reason, an exception is thrown within this method it will be * caught and the return value will be {@code ann.toString()}. * * @param ann the annotation to convert to a human readable String * @return a human readable String form of the annotation and it"s attributes */ public static String toString(Annotation ann) { try { Class<? extends Annotation> type = ann.annotationType(); StringBuilder builder = new StringBuilder(128); builder.append("@"); builder.append(type.getSimpleName()); boolean appendedAnyParameters = false; Method[] methods = type.getMethods(); for (Method method : methods) { if (!INHERITED_ANNOTATION_METHODS.contains(method.getName())) { Object defaultValue = method.getDefaultValue(); Object actualValue = method.invoke(ann); // If we have arrays, they have to be treated a little differently Object[] defaultArray = null, actualArray = null; if ( Object[].class.isAssignableFrom(method.getReturnType()) ) { defaultArray = (Object[]) defaultValue; actualArray = (Object[]) actualValue; } // Only print an attribute if it isn"t set to the default value if ( (defaultArray != null && !Arrays.equals(defaultArray, actualArray)) || (defaultArray == null && !actualValue.equals(defaultValue)) ) { if (appendedAnyParameters) { builder.append(", "); } else { builder.append("("); } builder.append(method.getName()); builder.append("="); if (actualArray != null) { builder.append( Arrays.toString(actualArray) ); } else { builder.append(actualValue); } appendedAnyParameters = true; } } } if (appendedAnyParameters) { builder.append(")"); } return builder.toString(); } catch (Exception e) { return ann.toString(); } } /** * Fetches all methods of all access types from the supplied class and super * classes. Methods that have been overridden in the inheritance hierarchy are * only returned once, using the instance lowest down the hierarchy. * * @param clazz the class to inspect * @return a collection of methods */ public static Collection<Method> getMethods(Class<?> clazz) { Collection<Method> found = new ArrayList<Method>(); while (clazz != null) { for (Method m1 : clazz.getDeclaredMethods()) { boolean overridden = false; for (Method m2 : found) { if ( m2.getName().equals(m1.getName()) && Arrays.deepEquals(m1.getParameterTypes(), m2.getParameterTypes())) { overridden = true; break; } } if (!overridden) found.add(m1); } clazz = clazz.getSuperclass(); } return found; } /** * Fetches all fields of all access types from the supplied class and super * classes. Fieldss that have been overridden in the inheritance hierarchy are * only returned once, using the instance lowest down the hierarchy. * * @param clazz the class to inspect * @return a collection of fields */ public static Collection<Field> getFields(Class<?> clazz) { Map<String,Field> fields = new HashMap<String, Field>(); while (clazz != null) { for (Field field : clazz.getDeclaredFields()) { if ( !fields.containsKey(field.getName()) ) { fields.put(field.getName(), field); } } clazz = clazz.getSuperclass(); } return fields.values(); } /** * Fetches the property descriptor for the named property of the supplied class. To * speed things up a cache is maintained of propertyName to PropertyDescriptor for * each class used with this method. If there is no property with the specified name, * returns null. * * @param clazz the class who"s properties to examine * @param property the String name of the property to look for * @return the PropertyDescriptor or null if none is found with a matching name */ public static PropertyDescriptor getPropertyDescriptor(Class<?> clazz, String property) { Map<String,PropertyDescriptor> pds = propertyDescriptors.get(clazz); if (pds == null) { try { BeanInfo info = Introspector.getBeanInfo(clazz); PropertyDescriptor[] descriptors = info.getPropertyDescriptors(); pds = new HashMap<String, PropertyDescriptor>(); for (PropertyDescriptor descriptor : descriptors) { pds.put(descriptor.getName(), descriptor); } propertyDescriptors.put(clazz, pds); } catch (IntrospectionException ie) { throw new RuntimeException("Could not examine class "" + clazz.getName() + "" using Introspector.getBeanInfo() to determine property information.", ie); } } return pds.get(property); } /** * <p>Attempts to find an accessible version of the method passed in, where accessible * is defined as the method itself being public and the declaring class being public. * Mostly useful as a workaround to the situation when * {@link PropertyDescriptor#getReadMethod()} and/or * {@link java.beans.PropertyDescriptor#getWriteMethod()} returns methods that are not * accessible (usually due to public implementations of interface methods in private * classes).

    *

*

Checks the method passed in and if it already meets these criteria it is returned * immediately. In general this leads to very little performance overhead

    *

*

If the method does not meet the criteria then the class" interfaces are scanned * for a matching method. If one is not found, then the class" superclass hierarchy * is searched. Finally, if no matching method can be found the original method is * returned.

    *
    * @param m a method that may or may not be accessible
    * @return either an accessible version of the same method, or the method passed in if
    *         an accessible version cannot be found
    */
   public static Method findAccessibleMethod(final Method m) {
       // If the passed in method is accessible, then just give it back.
       if (isPublic(m.getModifiers()) && isPublic(m.getDeclaringClass().getModifiers())) return m;
       if (m.isAccessible()) return m;
       final Class<?> clazz    = m.getDeclaringClass();
       final String name    = m.getName();
       final Class<?>[] ptypes = m.getParameterTypes();
       // Else, loop through the interfaces for the declaring class, looking for a
       // public version of the method that we can call
       for (Class<?> iface : clazz.getInterfaces()) {
           try {
               Method m2 = iface.getMethod(name, ptypes);
               if (m2.isAccessible()) return m2;
               if (isPublic(iface.getModifiers()) && isPublic(m2.getModifiers())) return m2;
           }
           catch (NoSuchMethodException nsme) { /* Not Unexpected. */ }
       }
       // Else loop through the superclasses looking for a public method
       Class<?> c = clazz.getSuperclass();
       while (c != null) {
           try {
               Method m2 = c.getMethod(name, ptypes);
               if (m2.isAccessible()) return m2;
               if (isPublic(c.getModifiers()) && isPublic(m2.getModifiers())) return m2;
           }
           catch (NoSuchMethodException nsme) { /* Not Unexpected. */ }
           c = c.getSuperclass();
       }
       // If we haven"t found anything at this point, just give up!
       return m;
   }

   /**
    * Looks for an instance (i.e. non-static) public field with the matching name and
    * returns it if one exists.  If no such field exists, returns null.
    *
    * @param clazz the clazz who"s fields to examine
    * @param property the name of the property/field to look for
    * @return the Field object or null if no matching field exists
    */
   public static Field getField(Class<?> clazz, String property) {
       try {
           Field field = clazz.getField(property);
           return !Modifier.isStatic(field.getModifiers()) ? field : null;
       }
       catch (NoSuchFieldException nsfe) {
           return null;
       }
   }
   /**
    * Returns an appropriate default value for the class supplied. Mirrors the defaults used
    * when the JVM initializes instance variables.
    *
    * @param clazz the class for which to find the default value
    * @return null for non-primitive types and an appropriate wrapper instance for primitives
    */
   public static Object getDefaultValue(Class<?> clazz) {
       if (clazz.isPrimitive()) {
           return primitiveDefaults.get(clazz);
       }
       else {
           return null;
       }
   }
   
   /**
    * Returns a set of all interfaces implemented by class supplied. This includes all
    * interfaces directly implemented by this class as well as those implemented by
    * superclasses or interface superclasses.
    * 
    * @param clazz
    * @return all interfaces implemented by this class
    */
   public static Set<Class<?>> getImplementedInterfaces(Class<?> clazz)
   {
       Set<Class<?>> interfaces = new HashSet<Class<?>>();
       
       if (clazz.isInterface())
           interfaces.add(clazz);
       while (clazz != null) {
           for (Class<?> iface : clazz.getInterfaces())
               interfaces.addAll(getImplementedInterfaces(iface));
           clazz = clazz.getSuperclass();
       } 
       return interfaces;
   }
   /**
    * Returns an array of Type objects representing the actual type arguments
    * to targetType used by clazz.
    * 
    * @param clazz the implementing class (or subclass)
    * @param targetType the implemented generic class or interface
    * @return an array of Type objects or null
    */
   public static Type[] getActualTypeArguments(Class<?> clazz, Class<?> targetType) {
       Set<Class<?>> classes = new HashSet<Class<?>>();
       classes.add(clazz);
       if (targetType.isInterface())
           classes.addAll(getImplementedInterfaces(clazz));
       Class<?> superClass = clazz.getSuperclass();
       while (superClass != null) {
           classes.add(superClass);
           superClass = superClass.getSuperclass();
       }
       for (Class<?> search : classes) {
           for (Type type : (targetType.isInterface() ? search.getGenericInterfaces()
                   : new Type[] { search.getGenericSuperclass() })) {
               if (type instanceof ParameterizedType) {
                   ParameterizedType parameterizedType = (ParameterizedType) type;
                   if (targetType.equals(parameterizedType.getRawType()))
                       return parameterizedType.getActualTypeArguments();
               }
           }
       }
       return null;
   }

}

 </source>
   
  
 
  







default values in an annotation.

   <source lang="java">
 

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; @Retention(RetentionPolicy.RUNTIME) @interface MyAnno {

 String str() default "Testing";
 int val() default 9000;

} class Meta3 {

 @MyAnno()
 public static void myMeth() {
   Meta3 ob = new Meta3();
   try {
     Class c = ob.getClass();
     Method m = c.getMethod("myMeth");
     MyAnno anno = m.getAnnotation(MyAnno.class);
     System.out.println(anno.str() + " " + anno.val());
   } catch (NoSuchMethodException exc) {
     System.out.println("Method Not Found.");
   }
 }
 public static void main(String args[]) {
   myMeth();
 }

}

 </source>
   
  
 
  







Does a method have an annotation

   <source lang="java">
 

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; @Retention(RetentionPolicy.RUNTIME) @interface MyMarker { } class Marker {

 @MyMarker
 public static void myMeth() {
   Marker ob = new Marker();
   try {
     Method m = ob.getClass().getMethod("myMeth");
     if (m.isAnnotationPresent(MyMarker.class))
       System.out.println("MyMarker is present.");
   } catch (NoSuchMethodException exc) {
     System.out.println("Method Not Found.");
   }
 }
 public static void main(String args[]) {
   myMeth();
 }

}

 </source>
   
  
 
  







Get annotation by annotation class

   <source lang="java">
 

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; @Retention(RetentionPolicy.RUNTIME) @interface MyAnno {

 String str();
 int val();

} class Meta {

 @MyAnno(str = "Two Parameters", val = 19)
 public static void myMeth(String str, int i) {
   Meta ob = new Meta();
   try {
     Class c = ob.getClass();
     Method m = c.getMethod("myMeth", String.class, int.class);
     MyAnno anno = m.getAnnotation(MyAnno.class);
     System.out.println(anno.str() + " " + anno.val());
   } catch (NoSuchMethodException exc) {
     System.out.println("Method Not Found.");
   }
 }
 public static void main(String args[]) {
   myMeth("test", 10);
 }

}

 </source>
   
  
 
  







Get Annotation Parameter

   <source lang="java">

// $Id: ReflectionHelper.java 16271 2009-04-07 20:20:12Z hardy.ferentschik $ /*

• JBoss, Home of Professional Open Source
• Copyright 2008, Red Hat Middleware LLC, and individual contributors
• by the @authors tag. See the copyright.txt in the distribution for a
• full listing of individual contributors.
• Licensed under the Apache License, Version 2.0 (the "License");
• you may not use this file except in compliance with the License.
• You may obtain a copy of the License at
• http://www.apache.org/licenses/LICENSE-2.0
• Unless required by applicable law or agreed to in writing, software
• distributed under the License is distributed on an "AS IS" BASIS,
• WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
• See the License for the specific language governing permissions and
• limitations under the License.
• /

import java.beans.Introspector; import java.lang.annotation.Annotation; import java.lang.reflect.AccessibleObject; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Member; import java.lang.reflect.Method; import java.lang.reflect.Modifier; import java.lang.reflect.ParameterizedType; import java.lang.reflect.Type; import java.lang.reflect.WildcardType; import java.util.ArrayList; import java.util.Arrays; import java.util.Iterator; import java.util.List; import java.util.Map;

/**

* Some reflection utility methods.
*
* @author Hardy Ferentschik
*/

public class ReflectionHelper {

 @SuppressWarnings("unchecked")
 public static <T> T getAnnotationParameter(Annotation annotation, String parameterName, Class<T> type) {
   try {
     Method m = annotation.getClass().getMethod( parameterName );
     Object o = m.invoke( annotation );
     if ( o.getClass().getName().equals( type.getName() ) ) {
       return ( T ) o;
     }
     else {
       String msg = "Wrong parameter type. Expected: " + type.getName() + " Actual: " + o.getClass().getName();
       throw new RuntimeException( msg );
     }
   }
   catch ( NoSuchMethodException e ) {
     String msg = "The specified annotation defines no parameter "" + parameterName + "".";
     throw new RuntimeException( msg, e );
   }
   catch ( IllegalAccessException e ) {
     String msg = "Unable to get "" + parameterName + "" from " + annotation.getClass().getName();
     throw new RuntimeException( msg, e );
   }
   catch ( InvocationTargetException e ) {
     String msg = "Unable to get "" + parameterName + "" from " + annotation.getClass().getName();
     throw new RuntimeException( msg, e );
   }
 }

}

 </source>
   
  
 
  







Show all annotations for a class and a method.

   <source lang="java">
 

import java.lang.annotation.Annotation; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; @Retention(RetentionPolicy.RUNTIME) @interface MyAnno {

 String str();
 int val();

} @Retention(RetentionPolicy.RUNTIME) @interface What {

 String description();

} @What(description = "An annotation test class") @MyAnno(str = "Meta2", val = 99) class Meta2 {

 @What(description = "An annotation test method")
 @MyAnno(str = "Testing", val = 100)
 public static void myMeth() {
   Meta2 ob = new Meta2();
   try {
     Annotation annos[] = ob.getClass().getAnnotations();
     System.out.println("All annotations for Meta2:");
     for (Annotation a : annos)
       System.out.println(a);
     Method m = ob.getClass().getMethod("myMeth");
     annos = m.getAnnotations();
     System.out.println("All annotations for myMeth:");
     for (Annotation a : annos)
       System.out.println(a);
   } catch (NoSuchMethodException exc) {
     System.out.println("Method Not Found.");
   }
 }
 public static void main(String args[]) {
   myMeth();
 }

}

 </source>
   
  
 
  







Uses reflection to display the annotation associated with a method.

   <source lang="java">
 

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; @Retention(RetentionPolicy.RUNTIME) @interface MyAnno {

 String str();
 int val();

} class Meta {

 @MyAnno(str = "Annotation Example", val = 100)
 public static void myMeth() {
   Meta ob = new Meta();
   try {
     Class c = ob.getClass();
     Method m = c.getMethod("myMeth");
     MyAnno anno = m.getAnnotation(MyAnno.class);
     System.out.println(anno.str() + " " + anno.val());
   } catch (NoSuchMethodException exc) {
     System.out.println("Method Not Found.");
   }
 }
 public static void main(String args[]) {
   myMeth();
 }

}

 </source>