Java/Reflection/Annotation

A better concise toString method for annotation types

 
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);
    }
    /**
     * <p>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:</p>
     *
     * <ul>
     *   <li>Uses the classes simple name instead of it"s fully qualified name.</li>
     *   <li>Only outputs attributes that are set to non-default values.</li>
     *
     * <p>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).</p>
     *
     * <p>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</p>
     *
     * <p>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.</p>
     *
     * @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;
    }
}

default values in an annotation.

  
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();
  }
}

Does a method have an annotation

  
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();
  }
}

Get annotation by annotation class

  
 
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);
  }
}

Get Annotation Parameter

 
// $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 );
    }
  }
}

Show all annotations for a class and a method.

  
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();
  }
}

Uses reflection to display the annotation associated with a method.

  
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();
  }
}