ResolverUtil.java
/*
* Copyright 2009-2021 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(Test, String)} 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
* @param <T>
* the generic type
*/
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.
*
* @param type
* the type
* @return true, if successful
*/
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 {
/** The parent. */
private Class<?> parent;
/**
* Constructs an IsA test using the supplied Class as the parent class/interface.
*
* @param parentType
* the parent type
*/
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 {
/** The annotation. */
private Class<? extends Annotation> annotation;
/**
* Constructs an AnnotatedWith test for the specified annotation type.
*
* @param annotation
* the annotation
*/
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
* @return the resolver util
*/
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
* @return the resolver util
*/
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}
* @return the resolver util
*/
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
* @return the package 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());
}
}
}