Java的动态加载及类加载器实践

在Java中，除了事先编写好代码完成执行代码，还可以从任何地方加载类。动态类加载允许程序在运行时动态编译和加载代码，打破了传统静态编译的限制。

主要的应用场景是插件系统、规则引擎、热部署等场景，本文将探讨其实现原理，并给出完整的最佳实践方案。

一、动态加载核心原理

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二、动态加载类代码

动态编译

// 该内容是字符串形式的类代码
String sourceCode = "public class DynamicDemo { /*...*/ }";
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
// DynamicDemo是字符串形式类代码的类名
JavaFileObject source = new StringJavaSource("DynamicDemo", sourceCode);// 使用内存文件管理器
MemoryJavaFileManager fileManager = new MemoryJavaFileManager(compiler.getStandardFileManager(null, null, null)
);// 执行编译任务
compiler.getTask(null, fileManager, null, null, null, Collections.singletonList(source)).call();

字节码内存管理

class MemoryJavaFileObject extends SimpleJavaFileObject {private final ByteArrayOutputStream bos = new ByteArrayOutputStream();@Overridepublic OutputStream openOutputStream() {return bos; // 字节码写入内存流}public byte[] getBytes() {return bos.toByteArray();}
}

类加载

public class MemoryClassLoader extends ClassLoader {private final Map<String, byte[]> classMap;@Overrideprotected Class<?> findClass(String name) {byte[] bytes = classMap.get(name);return defineClass(name, bytes, 0, bytes.length);}
}

利用反射调用

Class<?> clazz = classLoader.loadClass("DynamicDemo");
Object instance = clazz.newInstance();
Method method = clazz.getMethod("execute");
method.invoke(instance);

三、远程加载JAR到JVM

实现步骤

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远程JAR下载

public byte[] downloadRemoteJar(String jarUrl) throws IOException {try (InputStream in = new URL(jarUrl).openStream();ByteArrayOutputStream out = new ByteArrayOutputStream()) {byte[] buffer = new byte[4096];int bytesRead;while ((bytesRead = in.read(buffer)) != -1) {out.write(buffer, 0, bytesRead);}return out.toByteArray();}
}

内存JAR解析

public class JarInMemory {private final Map<String, byte[]> classEntries = new HashMap<>();private final Map<String, byte[]> resourceEntries = new HashMap<>();public JarInMemory(byte[] jarBytes) throws IOException {try (JarInputStream jis = new JarInputStream(new ByteArrayInputStream(jarBytes))) {JarEntry entry;while ((entry = jis.getNextJarEntry()) != null) {if (entry.isDirectory()) continue;ByteArrayOutputStream bos = new ByteArrayOutputStream();byte[] buffer = new byte[4096];int bytesRead;while ((bytesRead = jis.read(buffer)) != -1) {bos.write(buffer, 0, bytesRead);}byte[] data = bos.toByteArray();String name = entry.getName();if (name.endsWith(".class")) {String className = name.replace(".class", "").replace('/', '.');classEntries.put(className, data);} else {resourceEntries.put(name, data);}}}}
}

自定义类加载器

public class RemoteJarClassLoader extends ClassLoader {private final Map<String, byte[]> classMap;private final Map<String, byte[]> resourceMap;public RemoteJarClassLoader(ClassLoader parent, JarInMemory jar) {super(parent);this.classMap = new HashMap<>(jar.getClassEntries());this.resourceMap = new HashMap<>(jar.getResourceEntries());}@Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {byte[] classBytes = classMap.get(name);if (classBytes == null) {throw new ClassNotFoundException(name);}return defineClass(name, classBytes, 0, classBytes.length);}@Overridepublic InputStream getResourceAsStream(String name) {byte[] data = resourceMap.get(name);return data != null ? new ByteArrayInputStream(data) : null;}
}

四、更简洁的加载远程JAR

利用内置的URLClassLoader类加载器直接加载远程JAR，JVM 会按需下载 JAR 中的类文件，首次访问类时才会触发下载。加载的类遵循父委托机制，优先由父类加载器加载。

// 远程 JAR 地址
String jarUrl = "http://xxxxx.cn/abc.jar";// 创建 ClassLoader
URL[] urls = { new URL(jarUrl) };
try (URLClassLoader classLoader = new URLClassLoader(urls)) {// 加载类Class<?> clazz = classLoader.loadClass("cn.tworice.MyClass");// 实例化并调用方法Object instance = clazz.getDeclaredConstructor().newInstance();clazz.getMethod("myMethod").invoke(instance);
}