Java tutorial
/** * Copyright 2009-2019 the original author or authors. * * 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. */ package org.apache.ibatis.io; import java.io.IOException; import java.lang.annotation.Annotation; import java.util.HashSet; import java.util.List; import java.util.Set; import org.apache.ibatis.logging.Log; import org.apache.ibatis.logging.LogFactory; /** * <p>ResolverUtil is used to locate classes that are available in the/a class path and meet * arbitrary conditions. The two most common conditions are that a class implements/extends * another class, or that is it annotated with a specific annotation. However, through the use * of the {@link Test} class it is possible to search using arbitrary conditions.</p> * * <p>A ClassLoader is used to locate all locations (directories and jar files) in the class * path that contain classes within certain packages, and then to load those classes and * check them. By default the ClassLoader returned by * {@code Thread.currentThread().getContextClassLoader()} is used, but this can be overridden * by calling {@link #setClassLoader(ClassLoader)} prior to invoking any of the {@code find()} * methods.</p> * * <p>General searches are initiated by calling the * {@link #find(org.apache.ibatis.io.ResolverUtil.Test, String)} ()} method and supplying * a package name and a Test instance. This will cause the named package <b>and all sub-packages</b> * to be scanned for classes that meet the test. There are also utility methods for the common * use cases of scanning multiple packages for extensions of particular classes, or classes * annotated with a specific annotation.</p> * * <p>The standard usage pattern for the ResolverUtil class is as follows:</p> * * <pre> * ResolverUtil<ActionBean> resolver = new ResolverUtil<ActionBean>(); * resolver.findImplementation(ActionBean.class, pkg1, pkg2); * resolver.find(new CustomTest(), pkg1); * resolver.find(new CustomTest(), pkg2); * Collection<ActionBean> beans = resolver.getClasses(); * </pre> * * @author Tim Fennell */ public class ResolverUtil<T> { /* * An instance of Log to use for logging in this class. */ private static final Log log = LogFactory.getLog(ResolverUtil.class); /** * A simple interface that specifies how to test classes to determine if they * are to be included in the results produced by the ResolverUtil. */ public interface Test { /** * Will be called repeatedly with candidate classes. Must return True if a class * is to be included in the results, false otherwise. */ boolean matches(Class<?> type); } /** * A Test that checks to see if each class is assignable to the provided class. Note * that this test will match the parent type itself if it is presented for matching. */ public static class IsA implements Test { private Class<?> parent; /** Constructs an IsA test using the supplied Class as the parent class/interface. */ public IsA(Class<?> parentType) { this.parent = parentType; } /** Returns true if type is assignable to the parent type supplied in the constructor. */ @Override public boolean matches(Class<?> type) { return type != null && parent.isAssignableFrom(type); } @Override public String toString() { return "is assignable to " + parent.getSimpleName(); } } /** * A Test that checks to see if each class is annotated with a specific annotation. If it * is, then the test returns true, otherwise false. */ public static class AnnotatedWith implements Test { private Class<? extends Annotation> annotation; /** Constructs an AnnotatedWith test for the specified annotation type. */ public AnnotatedWith(Class<? extends Annotation> annotation) { this.annotation = annotation; } /** Returns true if the type is annotated with the class provided to the constructor. */ @Override public boolean matches(Class<?> type) { return type != null && type.isAnnotationPresent(annotation); } @Override public String toString() { return "annotated with @" + annotation.getSimpleName(); } } /** The set of matches being accumulated. */ private Set<Class<? extends T>> matches = new HashSet<>(); /** * The ClassLoader to use when looking for classes. If null then the ClassLoader returned * by Thread.currentThread().getContextClassLoader() will be used. */ private ClassLoader classloader; /** * Provides access to the classes discovered so far. If no calls have been made to * any of the {@code find()} methods, this set will be empty. * * @return the set of classes that have been discovered. */ public Set<Class<? extends T>> getClasses() { return matches; } /** * Returns the classloader that will be used for scanning for classes. If no explicit * ClassLoader has been set by the calling, the context class loader will be used. * * @return the ClassLoader that will be used to scan for classes */ public ClassLoader getClassLoader() { return classloader == null ? Thread.currentThread().getContextClassLoader() : classloader; } /** * Sets an explicit ClassLoader that should be used when scanning for classes. If none * is set then the context classloader will be used. * * @param classloader a ClassLoader to use when scanning for classes */ public void setClassLoader(ClassLoader classloader) { this.classloader = classloader; } /** * Attempts to discover classes that are assignable to the type provided. In the case * that an interface is provided this method will collect implementations. In the case * of a non-interface class, subclasses will be collected. Accumulated classes can be * accessed by calling {@link #getClasses()}. * * @param parent the class of interface to find subclasses or implementations of * @param packageNames one or more package names to scan (including subpackages) for classes */ public ResolverUtil<T> findImplementations(Class<?> parent, String... packageNames) { if (packageNames == null) { return this; } Test test = new IsA(parent); for (String pkg : packageNames) { find(test, pkg); } return this; } /** * Attempts to discover classes that are annotated with the annotation. Accumulated * classes can be accessed by calling {@link #getClasses()}. * * @param annotation the annotation that should be present on matching classes * @param packageNames one or more package names to scan (including subpackages) for classes */ public ResolverUtil<T> findAnnotated(Class<? extends Annotation> annotation, String... packageNames) { if (packageNames == null) { return this; } Test test = new AnnotatedWith(annotation); for (String pkg : packageNames) { find(test, pkg); } return this; } /** * Scans for classes starting at the package provided and descending into subpackages. * Each class is offered up to the Test as it is discovered, and if the Test returns * true the class is retained. Accumulated classes can be fetched by calling * {@link #getClasses()}. * * @param test an instance of {@link Test} that will be used to filter classes * @param packageName the name of the package from which to start scanning for * classes, e.g. {@code net.sourceforge.stripes} */ public ResolverUtil<T> find(Test test, String packageName) { String path = getPackagePath(packageName); try { List<String> children = VFS.getInstance().list(path); for (String child : children) { if (child.endsWith(".class")) { addIfMatching(test, child); } } } catch (IOException ioe) { log.error("Could not read package: " + packageName, ioe); } return this; } /** * Converts a Java package name to a path that can be looked up with a call to * {@link ClassLoader#getResources(String)}. * * @param packageName The Java package name to convert to a path */ protected String getPackagePath(String packageName) { return packageName == null ? null : packageName.replace('.', '/'); } /** * Add the class designated by the fully qualified class name provided to the set of * resolved classes if and only if it is approved by the Test supplied. * * @param test the test used to determine if the class matches * @param fqn the fully qualified name of a class */ @SuppressWarnings("unchecked") protected void addIfMatching(Test test, String fqn) { try { String externalName = fqn.substring(0, fqn.indexOf('.')).replace('/', '.'); ClassLoader loader = getClassLoader(); if (log.isDebugEnabled()) { log.debug("Checking to see if class " + externalName + " matches criteria [" + test + "]"); } Class<?> type = loader.loadClass(externalName); if (test.matches(type)) { matches.add((Class<T>) type); } } catch (Throwable t) { log.warn("Could not examine class '" + fqn + "'" + " due to a " + t.getClass().getName() + " with message: " + t.getMessage()); } } }