Java tutorial
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; } }