尝试在运行时从外部 JAR 加载类时出现 ClassNotFoundException

Question

我正在尝试从 JAR 加载一个在运行时表示为 byte[] 数组的类。
我知道关于要加载的类的两件事：

1.它实现了“RequiredInterface”
2. 我知道它的限定名称：“sample.TestJarLoadingClass”

我找到了solution，我必须在其中扩展 ClassLoader 但它会抛出：

Exception in thread "main" java.lang.ClassNotFoundException: sample.TestJarLoadingClass
    at java.lang.ClassLoader.findClass(ClassLoader.java:530)
    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 tasks.Main.main(Main.java:12)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

每当我想加载课程时。

这种情况可能是什么原因，我该如何摆脱这种情况？
任何帮助都非常感谢

主要方法：

public static void main(String[] args) throws IOException, ClassNotFoundException, IllegalAccessException, InstantiationException {
    Path path = Paths.get("src/main/java/tasks/sample.jar");
    RequiredInterface requiredInterface = (RequiredInterface) Class.forName("sample.TestJarLoadingClass", true, new ByteClassLoader(Files.readAllBytes(path))).newInstance();
}

自定义类加载器：

    import java.io.ByteArrayInputStream;
    import java.io.IOException;
    import java.io.InputStream;
    import java.util.Collections;
    import java.util.HashSet;
    import java.util.Set;
    import java.util.zip.ZipEntry;
    import java.util.zip.ZipInputStream;

    public class ByteClassLoader extends ClassLoader {
        private final byte[] jarBytes;
        private final Set<String> names;

        public ByteClassLoader(byte[] jarBytes) throws IOException {
            this.jarBytes = jarBytes;
            this.names = loadNames(jarBytes);
        }

        private Set<String> loadNames(byte[] jarBytes) throws IOException {
            Set<String> set = new HashSet<>();
            try (ZipInputStream jis = new ZipInputStream(new ByteArrayInputStream(jarBytes))) {
                ZipEntry entry;
                while ((entry = jis.getNextEntry()) != null) {
                    set.add(entry.getName());
                }
            }
            return Collections.unmodifiableSet(set);
        }

        @Override
        public InputStream getResourceAsStream(String resourceName) {
            if (!names.contains(resourceName)) {
                return null;
            }
            boolean found = false;
            ZipInputStream zipInputStream = null;
            try {
                zipInputStream = new ZipInputStream(new ByteArrayInputStream(jarBytes));
                ZipEntry entry;
                while ((entry = zipInputStream.getNextEntry()) != null) {
                    if (entry.getName().equals(resourceName)) {
                        found = true;
                        return zipInputStream;
                    }
                }
            } catch (IOException e) {;
                e.printStackTrace();
            } finally {
                if (zipInputStream != null && !found) {
                    try {
                        zipInputStream.close();
                    } catch (IOException e) {;
                        e.printStackTrace();
                    }
                }
            }
            return null;
        }
}

必需接口：

public interface RequiredInterface {
    String method();
}

JAR 文件中的类：

package sample;
public class TestJarLoadingClass implements RequiredInterface {
    @Override
    public String method() {
        return "works!";
    }
}

Answer 1

在我看来，我们这里有两个问题：

首先你应该重写包含加载类的实际逻辑的findClass 方法。这里的主要挑战是找到包含您的类的字节数组的一部分 - 因为您将整个 jar 作为字节数组，您需要使用 JarInputStream 来扫描您的类的字节数组。

但这可能还不够，因为您的 RequiredInterface 对于您的 ByteClassLoader 是未知的 - 所以您将能够读取类本身，但类的定义包含它实现 RequiredInterface 的信息，这是一个问题你的类加载器。这个很容易修复，您只需将常规类加载器作为构造函数参数传递给您的类并使用super(parentClassLoader)。

这是我的版本：

public class ByteClassLoader extends ClassLoader {
    private final byte[] jarBytes;

    public ByteClassLoader(ClassLoader parent, byte[] jarBytes) throws IOException {
        super(parent);
        this.jarBytes = jarBytes;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {

        // read byte array with JarInputStream
        try (JarInputStream jis = new JarInputStream(new ByteArrayInputStream(jarBytes))) {
            JarEntry nextJarEntry;

            // finding JarEntry will "move" JarInputStream at the begining of entry, so no need to create new input stream
            while ((nextJarEntry = jis.getNextJarEntry()) != null) {

                if (entryNameEqualsClassName(name, nextJarEntry)) {

                    // we need to know length of class to know how many bytes we should read
                    int classSize = (int) nextJarEntry.getSize();

                    // our buffer for class bytes
                    byte[] nextClass = new byte[classSize];

                    // actual reading
                    jis.read(nextClass, 0, classSize);

                    // create class from bytes
                    return defineClass(name, nextClass, 0, classSize, null);
                }
            }
            throw new ClassNotFoundException(String.format("Cannot find %s class", name));
        } catch (IOException e) {
            throw new ClassNotFoundException("Cannot read from jar input stream", e);
        }
    }


    private boolean entryNameEqualsClassName(String name, JarEntry nextJarEntry) {

        // removing .class suffix
        String entryName = nextJarEntry.getName().split("\\.")[0];

        // "convert" fully qualified name into path
        String className = name.replace(".", "/");

        return entryName.equals(className);
    }
}

及用法

    RequiredInterface requiredInterface = (RequiredInterface)Class.forName("com.sample.TestJarLoadingClass", true, new ByteClassLoader(ByteClassLoader.class.getClassLoader(), Files.readAllBytes(path))).newInstance();
    System.out.println(requiredInterface.method());

请注意，我的实现假定文件名 = 类名，因此例如不会找到非顶级类。当然，有些细节可能会更精致（比如异常处理）。