嘗試在運行時從外部JAR加載類時發生ClassNotFoundException

Question

我試圖在運行時從JAR加載一個表示為byte []數組的類。
我知道有關要加載的類的兩件事：

1.實現“ RequiredInterface”
2.我知道它的限定名稱：“ sample.TestJarLoadingClass”

我找到了必須擴展ClassLoader的解決方案 ，但它拋出：

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)

每當我想加載課程時。

造成這種情況的原因可能是什么？如何消除這種情況？
任何幫助，高度贊賞

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();
}

自定義類加載器：

    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;
        }
}

RequiredInterface：

public interface RequiredInterface {
    String method();
}

JAR文件中的類：

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

Answer 1

我認為這里有兩個問題：

首先，您應該重寫findClass方法，該方法包含加載類的實際邏輯。 這里的主要挑戰是找到包含您的類的字節數組的一部分-由於您將整個jar都作為字節數組，因此您將需要使用JarInputStream掃描類的字節數組。

但是因為你，這可能是不夠的RequiredInterface是您未知ByteClassLoader因此您將能夠讀取類本身，而是類的定義中包含的信息，它實現- RequiredInterface這是你的類加載器的問題。 這個很容易解決，您只需要將常規類加載器作為構造函數參數傳遞給您的super(parentClassLoader)並使用super(parentClassLoader) 。

這是我的版本：

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);
    }
}

和用法

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

請注意，我的實現假定文件名=類名，因此，例如，將找不到非頂級類。 當然，某些細節可能會更完善（例如異常處理）。