您可使用反射找到包中的全部类吗？

是否能够在给定的包中找到全部类或接口？ （快速查看例如Package ，彷佛没有。）

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#1楼

我整理了一个简单的github项目来解决这个问题：

https://github.com/ddopson/java-class-enumerator

它应同时适用于基于文件的类路径和jar文件。

若是您在签出项目后运行“ make”，它将打印出来：

Cleaning...
rm -rf build/
 Building...
javac -d build/classes src/pro/ddopson/ClassEnumerator.java src/test/ClassIShouldFindOne.java src/test/ClassIShouldFindTwo.java src/test/subpkg/ClassIShouldFindThree.java src/test/TestClassEnumeration.java
 Making JAR Files...
jar cf build/ClassEnumerator_test.jar -C build/classes/ . 
jar cf build/ClassEnumerator.jar -C build/classes/ pro
 Running Filesystem Classpath Test...
java -classpath build/classes test.TestClassEnumeration
ClassDiscovery: Package: 'test' becomes Resource: 'file:/Users/Dopson/work/other/java-class-enumeration/build/classes/test'
ClassDiscovery: Reading Directory '/Users/Dopson/work/other/java-class-enumeration/build/classes/test'
ClassDiscovery: FileName 'ClassIShouldFindOne.class'  =>  class 'test.ClassIShouldFindOne'
ClassDiscovery: FileName 'ClassIShouldFindTwo.class'  =>  class 'test.ClassIShouldFindTwo'
ClassDiscovery: FileName 'subpkg'  =>  class 'null'
ClassDiscovery: Reading Directory '/Users/Dopson/work/other/java-class-enumeration/build/classes/test/subpkg'
ClassDiscovery: FileName 'ClassIShouldFindThree.class'  =>  class 'test.subpkg.ClassIShouldFindThree'
ClassDiscovery: FileName 'TestClassEnumeration.class'  =>  class 'test.TestClassEnumeration'
 Running JAR Classpath Test...
java -classpath build/ClassEnumerator_test.jar  test.TestClassEnumeration
ClassDiscovery: Package: 'test' becomes Resource: 'jar:file:/Users/Dopson/work/other/java-class-enumeration/build/ClassEnumerator_test.jar!/test'
ClassDiscovery: Reading JAR file: '/Users/Dopson/work/other/java-class-enumeration/build/ClassEnumerator_test.jar'
ClassDiscovery: JarEntry 'META-INF/'  =>  class 'null'
ClassDiscovery: JarEntry 'META-INF/MANIFEST.MF'  =>  class 'null'
ClassDiscovery: JarEntry 'pro/'  =>  class 'null'
ClassDiscovery: JarEntry 'pro/ddopson/'  =>  class 'null'
ClassDiscovery: JarEntry 'pro/ddopson/ClassEnumerator.class'  =>  class 'null'
ClassDiscovery: JarEntry 'test/'  =>  class 'null'
ClassDiscovery: JarEntry 'test/ClassIShouldFindOne.class'  =>  class 'test.ClassIShouldFindOne'
ClassDiscovery: JarEntry 'test/ClassIShouldFindTwo.class'  =>  class 'test.ClassIShouldFindTwo'
ClassDiscovery: JarEntry 'test/subpkg/'  =>  class 'null'
ClassDiscovery: JarEntry 'test/subpkg/ClassIShouldFindThree.class'  =>  class 'test.subpkg.ClassIShouldFindThree'
ClassDiscovery: JarEntry 'test/TestClassEnumeration.class'  =>  class 'test.TestClassEnumeration'
 Tests Passed.

另请参阅个人其余答案

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#2楼

Google Guava 14包含一个新的ClassPath类，该类具备三种方法来扫描顶级类：

• getTopLevelClasses()
• getTopLevelClasses(String packageName)
• getTopLevelClassesRecursive(String packageName)

有关更多信息，请参见ClassPath javadocs 。

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#3楼

一般，类加载器不容许扫描类路径上的全部类。 可是一般惟一使用的类加载器是UrlClassLoader，咱们能够从中检索目录和jar文件的列表（请参阅getURLs ），并一一打开它们以列出可用的类。 这种方法称为类路径扫描，是在Scannotation and Reflections中实现的 。

Reflections reflections = new Reflections("my.package");
Set<Class<? extends Object>> classes = reflections.getSubTypesOf(Object.class);

另外一种方法是使用Java Pluggable Annotation Processing API编写注释处理器，该处理器将在编译时收集全部带注释的类并构建索引文件以供运行时使用。 此机制在ClassIndex库中实现：

// package-info.java
@IndexSubclasses
package my.package;

// your code
Iterable<Class> classes = ClassIndex.getPackageClasses("my.package");

请注意，因为Java编译器自动发现了在类路径上找到的任何处理器，所以扫描是彻底自动化的，所以不须要其余设置。

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#4楼

您须要在类路径中查找每一个类加载器条目：

String pkg = "org/apache/commons/lang";
    ClassLoader cl = ClassLoader.getSystemClassLoader();
    URL[] urls = ((URLClassLoader) cl).getURLs();
    for (URL url : urls) {
        System.out.println(url.getFile());
        File jar = new File(url.getFile());
        // ....
    }

若是条目是目录，则只需在右侧的子目录中查找：

if (jar.isDirectory()) {
    File subdir = new File(jar, pkg);
    if (!subdir.exists())
        continue;
    File[] files = subdir.listFiles();
    for (File file : files) {
        if (!file.isFile())
            continue;
        if (file.getName().endsWith(".class"))
            System.out.println("Found class: "
                    + file.getName().substring(0,
                            file.getName().length() - 6));
    }
}

若是条目是文件，而且是jar，请检查其ZIP条目：

else {
    // try to open as ZIP
    try {
        ZipFile zip = new ZipFile(jar);
        for (Enumeration<? extends ZipEntry> entries = zip
                .entries(); entries.hasMoreElements();) {
            ZipEntry entry = entries.nextElement();
            String name = entry.getName();
            if (!name.startsWith(pkg))
                continue;
            name = name.substring(pkg.length() + 1);
            if (name.indexOf('/') < 0 && name.endsWith(".class"))
                System.out.println("Found class: "
                        + name.substring(0, name.length() - 6));
        }
    } catch (ZipException e) {
        System.out.println("Not a ZIP: " + e.getMessage());
    } catch (IOException e) {
        System.err.println(e.getMessage());
    }
}

如今，一旦有了全部带有包的类名，就能够尝试使用反射加载它们，并分析它们是类仍是接口等。

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#5楼

我一直在尝试使用Reflections库，可是在使用它时遇到了一些问题，我应该包括的jar太多了，只是为了简单地获取包中的类。

我将发布在这个重复的问题中找到的解决方案： 如何获取包中的全部类名称？

答案是由sp00m编写的 ； 我添加了一些更正使其生效：

import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.util.Enumeration;
import java.util.LinkedList;
import java.util.List;

public final class ClassFinder {

    private final static char DOT = '.';
    private final static char SLASH = '/';
    private final static String CLASS_SUFFIX = ".class";
    private final static String BAD_PACKAGE_ERROR = "Unable to get resources from path '%s'. Are you sure the given '%s' package exists?";

    public final static List<Class<?>> find(final String scannedPackage) {
        final ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        final String scannedPath = scannedPackage.replace(DOT, SLASH);
        final Enumeration<URL> resources;
        try {
            resources = classLoader.getResources(scannedPath);
        } catch (IOException e) {
            throw new IllegalArgumentException(String.format(BAD_PACKAGE_ERROR, scannedPath, scannedPackage), e);
        }
        final List<Class<?>> classes = new LinkedList<Class<?>>();
        while (resources.hasMoreElements()) {
            final File file = new File(resources.nextElement().getFile());
            classes.addAll(find(file, scannedPackage));
        }
        return classes;
    }

    private final static List<Class<?>> find(final File file, final String scannedPackage) {
        final List<Class<?>> classes = new LinkedList<Class<?>>();
        if (file.isDirectory()) {
            for (File nestedFile : file.listFiles()) {
                classes.addAll(find(nestedFile, scannedPackage));
            }
        //File names with the $1, $2 holds the anonymous inner classes, we are not interested on them. 
        } else if (file.getName().endsWith(CLASS_SUFFIX) && !file.getName().contains("$")) {

            final int beginIndex = 0;
            final int endIndex = file.getName().length() - CLASS_SUFFIX.length();
            final String className = file.getName().substring(beginIndex, endIndex);
            try {
                final String resource = scannedPackage + DOT + className;
                classes.add(Class.forName(resource));
            } catch (ClassNotFoundException ignore) {
            }
        }
        return classes;
    }

}

要使用它，只需像本例中提到的sp00n那样调用find方法：若是须要，我添加了类实例的建立。

List<Class<?>> classes = ClassFinder.find("com.package");

ExcelReporting excelReporting;
for (Class<?> aClass : classes) {
    Constructor constructor = aClass.getConstructor();
    //Create an object of the class type
    constructor.newInstance();
    //...
}