我正在编写一个Maven插件，我需要知道List<Person>的类型，其中Person是依赖项中定义的对象。此插件在process-classes阶段运行，以根据在宿主项目中的注释类中找到的内容生成一些文件。在这种情况下，宿主项目包含对其他项目中的库的引用，这些库作为Maven依赖项包含在内。
Person类：
package com.dependency.models;
public class Person {
// Irrelevent
}
在我正在执行查询的类中使用：
package com.project.wrappers;
import com.dependency.Person;
@MyAnnotation
public class Wrapper {
List<Person> people;
// Other irrelevant stuff
}
注释：
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(value={ElementType.TYPE})
public @interface MyAnnotation {}
在Maven插件中使用上下文我通过注释加载所有Wrapper个对象，然后进行处理。为此，我需要通过Reflections库使用自定义类加载器：
public static Set<Class<?>> findAnnotatedClasses(MavenProject mavenProject, Class<? extends Annotation> input) throws MojoExecutionException {
List<String> classpathElements = null;
try {
classpathElements = mavenProject.getCompileClasspathElements();
List<URL> projectClasspathList = new ArrayList<URL>();
for (String element : classpathElements) {
Application.getLogger().debug("Considering compile classpath element (via MavenProject): " + element);
try {
projectClasspathList.add(new File(element).toURI().toURL());
} catch (MalformedURLException e) {
throw new MojoExecutionException(element + " is an invalid classpath element", e);
}
}
// Retain annotations
JavassistAdapter javassistAdapter = new JavassistAdapter();
javassistAdapter.includeInvisibleTag = false;
URLClassLoader urlClassLoader = new URLClassLoader(projectClasspathList.toArray(new URL[]{}),
Thread.currentThread().getContextClassLoader());
Reflections reflections = new Reflections(
new ConfigurationBuilder().setUrls(
ClasspathHelper.forClassLoader(urlClassLoader)
).addClassLoader(urlClassLoader).setScanners(new TypeAnnotationsScanner(), new TypeElementsScanner(),
new FieldAnnotationsScanner(), new TypeAnnotationsScanner(), new SubTypesScanner(false)
).setMetadataAdapter(javassistAdapter)
);
return findAnnotatedClasses(reflections, input);
} catch (DependencyResolutionRequiredException e) {
throw new MojoExecutionException("Dependency resolution failed", e);
}
}
到这里的一切都很棒。但如果我在我的解析器中尝试以下操作，则会失败：
Field field = Wrapper.class.getDeclaredField("people");
ParameterizedType listType = (ParameterizedType) field.getGenericType();
Class<?> listTypeClass = (Class<?>) listType.getActualTypeArguments()[0];
抛出以下异常：
Caused by: java.lang.TypeNotPresentException: Type com.dependency.models.Person not present
at sun.reflect.generics.factory.CoreReflectionFactory.makeNamedType(CoreReflectionFactory.java:117)
at sun.reflect.generics.visitor.Reifier.visitClassTypeSignature(Reifier.java:125)
at sun.reflect.generics.tree.ClassTypeSignature.accept(ClassTypeSignature.java:49)
at sun.reflect.generics.visitor.Reifier.reifyTypeArguments(Reifier.java:68)
at sun.reflect.generics.visitor.Reifier.visitClassTypeSignature(Reifier.java:138)
at sun.reflect.generics.tree.ClassTypeSignature.accept(ClassTypeSignature.java:49)
at sun.reflect.generics.repository.FieldRepository.getGenericType(FieldRepository.java:85)
at java.lang.reflect.Field.getGenericType(Field.java:247)
如果进一步追随，那么：
Caused by: java.lang.ClassNotFoundException: com.dependency.models.Person
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at sun.reflect.generics.factory.CoreReflectionFactory.makeNamedType(CoreReflectionFactory.java:114)
这意味着我的类加载适用于最后一步之前的所有内容。我想我需要一种方法来覆盖sun.reflect库中使用的类加载器，但这可能是不可能的。有关更好方法的任何建议吗？
更新
我已将依赖关系jar添加到URLClassLoader并尝试覆盖上面CoreReflectionFactory使用的当前类加载器无效。
答案 0 :(得分：0)
解决方案不仅是将依赖项添加到类加载器，而且还要实际解析jar路径，然后将这些jar的路径添加到classpath中，以便反射库将其包含在其扫描中。
因此，从Maven获得所有工件，然后在构建该工件时进行解析并将每条路径添加到Reflections。像这样：
public static URL[] buildProjectClasspathList(ArtifactReference artifactReference) throws ArtifactResolutionException, MalformedURLException, DependencyResolutionRequiredException {
List<URL> projectClasspathList = new ArrayList<URL>();
// Load build class path
if(classpathElementsCache == null) {
Application.getLogger().debug("Compile Classpath Elements Cache was null; fetching update");
classpathElementsCache = artifactReference.getMavenProject().getCompileClasspathElements();
}
for (String element : classpathElementsCache) {
Application.getLogger().debug("Looking at compile classpath element (via MavenProject): " + element);
Application.getLogger().debug(" Adding: " + element);
projectClasspathList.add(new File(element).toURI().toURL());
}
// Load artifact(s) jars using resolver
if(dependencyArtifactsCache == null) {
Application.getLogger().debug("Dependency Artifacts Cache was null; fetching update");
dependencyArtifactsCache = artifactReference.getMavenProject().getDependencyArtifacts();
}
Application.getLogger().debug("Number of artifacts to resolve: "
+ dependencyArtifactsCache.size());
for (Artifact unresolvedArtifact : dependencyArtifactsCache) {
String artifactId = unresolvedArtifact.getArtifactId();
if (!isArtifactResolutionRequired(unresolvedArtifact, artifactReference)) {
Application.getLogger().debug(" Skipping: " + unresolvedArtifact.toString());
continue;
}
org.eclipse.aether.artifact.Artifact aetherArtifact = new DefaultArtifact(
unresolvedArtifact.getGroupId(),
unresolvedArtifact.getArtifactId(),
unresolvedArtifact.getClassifier(),
unresolvedArtifact.getType(),
unresolvedArtifact.getVersion());
ArtifactRequest artifactRequest = new ArtifactRequest()
.setRepositories(artifactReference.getRepositories())
.setArtifact(aetherArtifact);
// This takes time; minimizing what needs to be resolved is the goal of the specified dependency code block
ArtifactResult resolutionResult = artifactReference.getRepoSystem()
.resolveArtifact(artifactReference.getRepoSession(), artifactRequest);
// The file should exist, but we never know.
File file = resolutionResult.getArtifact().getFile();
if (file == null || !file.exists()) {
Application.getLogger().warn("Artifact " + artifactId +
" has no attached file. Its content will not be copied in the target model directory.");
continue;
}
String jarPath = "jar:file:" + file.getAbsolutePath() + "!/";
Application.getLogger().debug("Adding resolved artifact: " + file.getAbsolutePath());
projectClasspathList.add(new URL(jarPath));
}
return projectClasspathList.toArray(new URL[]{});
}
ArtifactReference包含MavenProject和Repo对象的地方：
public class ArtifactReference {
private MavenProject mavenProject;
private RepositorySystem repoSystem;
private RepositorySystemSession repoSession;
private List<RemoteRepository> repositories;
private List<String> specifiedDependencies;
}