1、Agent启动流程

找到入口方法SkyWalkingAgent的premain()方法，源码如下：

public class SkyWalkingAgent {
  
    /**
     * Main entrance. Use byte-buddy transform to enhance all classes, which define in plugins.
     * -javaagent:/path/to/agent.jar=agentArgs
     * -javaagent:/path/to/agent.jar=k1=v1,k2=v2...
     * 等号之后都是参数,也就是入参agentArgs
     * -javaagent参数必须在-jar之前
     */
    public static void premain(String agentArgs, Instrumentation instrumentation) throws PluginException {
        final PluginFinder pluginFinder;
        try {
            // 1.初始化配置
            SnifferConfigInitializer.initializeCoreConfig(agentArgs);
        } catch (Exception e) {
            // try to resolve a new logger, and use the new logger to write the error log here
            LogManager.getLogger(SkyWalkingAgent.class)
                    .error(e, "SkyWalking agent initialized failure. Shutting down.");
            return;
        } finally {
            // refresh logger again after initialization finishes
            LOGGER = LogManager.getLogger(SkyWalkingAgent.class);
        }

        try {
            // 2.加载插件
            pluginFinder = new PluginFinder(new PluginBootstrap().loadPlugins());
        } catch (AgentPackageNotFoundException ape) {
            LOGGER.error(ape, "Locate agent.jar failure. Shutting down.");
            return;
        } catch (Exception e) {
            LOGGER.error(e, "SkyWalking agent initialized failure. Shutting down.");
            return;
        }

        // 3.定制化Agent行为
        final ByteBuddy byteBuddy = new ByteBuddy().with(TypeValidation.of(Config.Agent.IS_OPEN_DEBUGGING_CLASS));

        AgentBuilder agentBuilder = new AgentBuilder.Default(byteBuddy).ignore(
                nameStartsWith("net.bytebuddy.")
                        .or(nameStartsWith("org.slf4j."))
                        .or(nameStartsWith("org.groovy."))
                        .or(nameContains("javassist"))
                        .or(nameContains(".asm."))
                        .or(nameContains(".reflectasm."))
                        .or(nameStartsWith("sun.reflect"))
                        .or(allSkyWalkingAgentExcludeToolkit())
                        .or(ElementMatchers.isSynthetic()));

        JDK9ModuleExporter.EdgeClasses edgeClasses = new JDK9ModuleExporter.EdgeClasses();
        try {
            agentBuilder = BootstrapInstrumentBoost.inject(pluginFinder, instrumentation, agentBuilder, edgeClasses);
        } catch (Exception e) {
            LOGGER.error(e, "SkyWalking agent inject bootstrap instrumentation failure. Shutting down.");
            return;
        }

        try {
            agentBuilder = JDK9ModuleExporter.openReadEdge(instrumentation, agentBuilder, edgeClasses);
        } catch (Exception e) {
            LOGGER.error(e, "SkyWalking agent open read edge in JDK 9+ failure. Shutting down.");
            return;
        }

        if (Config.Agent.IS_CACHE_ENHANCED_CLASS) {
            try {
                agentBuilder = agentBuilder.with(new CacheableTransformerDecorator(Config.Agent.CLASS_CACHE_MODE));
                LOGGER.info("SkyWalking agent class cache [{}] activated.", Config.Agent.CLASS_CACHE_MODE);
            } catch (Exception e) {
                LOGGER.error(e, "SkyWalking agent can't active class cache.");
            }
        }

        agentBuilder.type(pluginFinder.buildMatch())
                    .transform(new Transformer(pluginFinder))
                    .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
                    .with(new RedefinitionListener())
                    .with(new Listener())
                    .installOn(instrumentation);

        try {
            // 4.启动服务
            ServiceManager.INSTANCE.boot();
        } catch (Exception e) {
            LOGGER.error(e, "Skywalking agent boot failure.");
        }

        // 5.注册关闭钩子
        Runtime.getRuntime()
                .addShutdownHook(new Thread(ServiceManager.INSTANCE::shutdown, "skywalking service shutdown thread"));
    }  

小结：

2、Agent配置加载流程

Agent打包目录下的config目录中有个agent.config的文件，这就是Agent的配置文件

# 配置模块.配置项
agent.service_name = crmApp
collector.backend_service = 127.0.0.1:8080
logging.level=info

premain()中调用SnifferConfigInitializer的initializeCoreConfig()方法初始化配置，源码如下：

public class SnifferConfigInitializer {

    public static void initializeCoreConfig(String agentOptions) {
        // 加载配置信息 优先级:agent参数 > 系统环境变量 > /config/agent.config
        AGENT_SETTINGS = new Properties();
        // /config/agent.config
        try (final InputStreamReader configFileStream = loadConfig()) {
            AGENT_SETTINGS.load(configFileStream);
            for (String key : AGENT_SETTINGS.stringPropertyNames()) {
                String value = (String) AGENT_SETTINGS.get(key);
                // 配置值里的占位符替换
                // aaa = xxx
                // bbb = ${aaa}-yyy => xxx-yyy
                AGENT_SETTINGS.put(key, PropertyPlaceholderHelper.INSTANCE.replacePlaceholders(value, AGENT_SETTINGS));
            }

        } catch (Exception e) {
            LOGGER.error(e, "Failed to read the config file, skywalking is going to run in default config.");
        }

        try {
            // 系统环境变量
            overrideConfigBySystemProp();
        } catch (Exception e) {
            LOGGER.error(e, "Failed to read the system properties.");
        }

        // agent参数
        agentOptions = StringUtil.trim(agentOptions, ',');
        if (!StringUtil.isEmpty(agentOptions)) {
            try {
                agentOptions = agentOptions.trim();
                LOGGER.info("Agent options is {}.", agentOptions);

                overrideConfigByAgentOptions(agentOptions);
            } catch (Exception e) {
                LOGGER.error(e, "Failed to parse the agent options, val is {}.", agentOptions);
            }
        }

        // 1)将配置信息映射到Config类
        initializeConfig(Config.class);
        // reconfigure logger after config initialization
        // 根据配置信息重新指定日志解析器
        configureLogger();
        LOGGER = LogManager.getLogger(SnifferConfigInitializer.class);

        // 检查agent名称和后端地址是否配置
        if (StringUtil.isEmpty(Config.Agent.SERVICE_NAME)) {
            throw new ExceptionInInitializerError("`agent.service_name` is missing.");
        }
        if (StringUtil.isEmpty(Config.Collector.BACKEND_SERVICE)) {
            throw new ExceptionInInitializerError("`collector.backend_service` is missing.");
        }
        if (Config.Plugin.PEER_MAX_LENGTH <= 3) {
            LOGGER.warn(
                "PEER_MAX_LENGTH configuration:{} error, the default value of 200 will be used.",
                Config.Plugin.PEER_MAX_LENGTH
            );
            Config.Plugin.PEER_MAX_LENGTH = 200;
        }

        // 标记配置加载完成
        IS_INIT_COMPLETED = true;
    }
  
    /**
     * 根据配置信息重新指定日志解析器 JSON和PATTERN两种日志格式
     */
    static void configureLogger() {
        switch (Config.Logging.RESOLVER) {
            case JSON:
                LogManager.setLogResolver(new JsonLogResolver());
                break;
            case PATTERN:
            default:
                LogManager.setLogResolver(new PatternLogResolver());
        }
    }  

代码1)处将配置信息映射到Config类，Config类结构如下，Config类中就包含Agent所有的配置，可以看到一共有11个配置模块，每个配置模块下再是具体的配置项，和agent.config中指定的配置结构是相同的

小结：

3、自定义类加载器AgentClassLoader

插件加载的第一步会去初始化自定义类加载器AgentClassLoader，所以我们先来看到AgentClassLoader的源码

1）、类加载器的并行加载模式

AgentClassLoader的静态代码块里调用ClassLoader的registerAsParallelCapable()方法

/**
 * The <code>AgentClassLoader</code> represents a classloader, which is in charge of finding plugins and interceptors.
 * 自定义类加载器,负责查找插件和拦截器
 */
public class AgentClassLoader extends ClassLoader {

    static {
        /*
         * Try to solve the classloader dead lock. See https://github.com/apache/skywalking/pull/2016
         * 为了解决ClassLoader死锁问题,开启类加载器的并行加载模式
         */
        registerAsParallelCapable();
    }

先说下，什么是具备并行能力的类加载器？

在JDK 1.7之前，类加载器在加载类的时候是串行加载的，比如有100个类需要加载，那么就排队，加载完上一个再加载下一个，这样加载效率就很低

在JDK 1.7之后，就提供了类加载器并行能力，就是把锁的粒度变小，之前ClassLoader加载类的时候加锁的时候是用自身作为锁的

接下来我们一起来看ClassLoader的registerAsParallelCapable()方法源码：

public abstract class ClassLoader {

    /**
     * 将调用该方法的类加载器注册为具备并行能力的
     * 同时满足以下两个条件时,注册才会成功
     * 1.调用该方法的类加载器实例还未创建
     * 2.调用该方法的类加载器所有父类(Object类除外)都注册为具备并行能力的
     */
    @CallerSensitive
    protected static boolean registerAsParallelCapable() {
      	// 把调用该方法的Class对象转换为ClassLoader的子类
        Class<? extends ClassLoader> callerClass =
                Reflection.getCallerClass().asSubclass(ClassLoader.class);
      	// 注册为具备并行能力的
        return ParallelLoaders.register(callerClass);
    }

    private static class ParallelLoaders {
        private ParallelLoaders() {}

        // loaderTypes中保存了所有具备并行能力的类加载器
        private static final Set<Class<? extends ClassLoader>> loaderTypes =
                Collections.newSetFromMap(new WeakHashMap<>());
        static {
            // ClassLoader本身就是支持并行加载的
            synchronized (loaderTypes) { loaderTypes.add(ClassLoader.class); }
        }

        static boolean register(Class<? extends ClassLoader> c) {
            synchronized (loaderTypes) {
                // 当且仅当该类加载器的所有父类都具备并行能力时,该类加载器才能被注册成功
                if (loaderTypes.contains(c.getSuperclass())) {
                    loaderTypes.add(c);
                    return true;
                } else {
                    return false;
                }
            }
        }

        /**
         * 判定给定的类加载器是否具备并行能力
         */
        static boolean isRegistered(Class<? extends ClassLoader> c) {
            synchronized (loaderTypes) {
                return loaderTypes.contains(c);
            }
        }

并行加载的实现我们一起来看下ClassLoader的loadClass()方法源码：

public abstract class ClassLoader {
  
  	private final ConcurrentHashMap<String, Object> parallelLockMap;

    private ClassLoader(Void unused, String name, ClassLoader parent) {
        this.name = name;
        this.parent = parent;
        this.unnamedModule = new Module(this);
      	// 判断当前类加载器是否具备并行能力,如果具备则对parallelLockMap进行初始化
        if (ParallelLoaders.isRegistered(this.getClass())) {
            parallelLockMap = new ConcurrentHashMap<>();
            package2certs = new ConcurrentHashMap<>();
            assertionLock = new Object();
        } else {
            // no finer-grained lock; lock on the classloader instance
            parallelLockMap = null;
            package2certs = new Hashtable<>();
            assertionLock = this;
        }
        this.nameAndId = nameAndId(this);
    }
  
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
      	// 加锁,调用getClassLoadingLock方法获取类加载时的锁
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }
 
    protected Object getClassLoadingLock(String className) {
      	// 默认锁是自身
        Object lock = this;
        if (parallelLockMap != null) {
            Object newLock = new Object();
          	// k:要加载的类名 v:新的锁
            lock = parallelLockMap.putIfAbsent(className, newLock);
          	// 如果是第一次put,则返回newLock
            if (lock == null) {
                lock = newLock;
            }
        }
        return lock;
    }  

并行加载的原理：

如果当前类加载器是支持并行加载的，就把加载类时锁的粒度降低到加载的具体的某一个类上，而不是锁掉整个类加载器

2）、AgentClassLoader源码解析

/**
 * The <code>AgentClassLoader</code> represents a classloader, which is in charge of finding plugins and interceptors.
 * 自定义类加载器,负责查找插件和拦截器
 */
public class AgentClassLoader extends ClassLoader {

    static {
        /*
         * Try to solve the classloader dead lock. See https://github.com/apache/skywalking/pull/2016
         * 为了解决ClassLoader死锁问题,开启类加载器的并行加载模式
         */
        registerAsParallelCapable();
    }

    private static final ILog LOGGER = LogManager.getLogger(AgentClassLoader.class);
    /**
     * The default class loader for the agent.
     */
    private static AgentClassLoader DEFAULT_LOADER;

    private List<File> classpath;
    private List<Jar> allJars;
    private ReentrantLock jarScanLock = new ReentrantLock();

    public static AgentClassLoader getDefault() {
        return DEFAULT_LOADER;
    }

    /**
     * Init the default class loader.
     * 初始化默认的类加载器
     *
     * @throws AgentPackageNotFoundException if agent package is not found.
     */
    public static void initDefaultLoader() throws AgentPackageNotFoundException {
        if (DEFAULT_LOADER == null) {
            synchronized (AgentClassLoader.class) {
                if (DEFAULT_LOADER == null) {
                    // AgentClassLoader的父类加载器是加载PluginBootstrap的类加载器(AppClassLoader)
                    DEFAULT_LOADER = new AgentClassLoader(PluginBootstrap.class.getClassLoader());
                }
            }
        }
    }

    public AgentClassLoader(ClassLoader parent) throws AgentPackageNotFoundException {
        super(parent);
        // 获取agent主目录
        File agentDictionary = AgentPackagePath.getPath();
        classpath = new LinkedList<>();
        // 默认agent主目录下的plugins和activations目录作为AgentClassLoader的classpath
        // 只有这两个目录下的jar包才会被AgentClassLoader所加载
        Config.Plugin.MOUNT.forEach(mountFolder -> classpath.add(new File(agentDictionary, mountFolder)));
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        List<Jar> allJars = getAllJars();
        String path = name.replace('.', '/').concat(".class");
        for (Jar jar : allJars) {
            JarEntry entry = jar.jarFile.getJarEntry(path);
            if (entry == null) {
                continue;
            }
            try {
                URL classFileUrl = new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/" + path);
                byte[] data;
                try (final BufferedInputStream is = new BufferedInputStream(
                    classFileUrl.openStream()); final ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
                    int ch;
                    while ((ch = is.read()) != -1) {
                        baos.write(ch);
                    }
                    data = baos.toByteArray();
                }
                return processLoadedClass(defineClass(name, data, 0, data.length));
            } catch (IOException e) {
                LOGGER.error(e, "find class fail.");
            }
        }
        throw new ClassNotFoundException("Can't find " + name);
    }

    @Override
    protected URL findResource(String name) {
        List<Jar> allJars = getAllJars();
        for (Jar jar : allJars) {
            JarEntry entry = jar.jarFile.getJarEntry(name);
            if (entry != null) {
                try {
                    return new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/" + name);
                } catch (MalformedURLException ignored) {
                }
            }
        }
        return null;
    }

    @Override
    protected Enumeration<URL> findResources(String name) throws IOException {
        List<URL> allResources = new LinkedList<>();
        List<Jar> allJars = getAllJars();
        for (Jar jar : allJars) {
            JarEntry entry = jar.jarFile.getJarEntry(name);
            if (entry != null) {
                allResources.add(new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/" + name));
            }
        }

        final Iterator<URL> iterator = allResources.iterator();
        return new Enumeration<URL>() {
            @Override
            public boolean hasMoreElements() {
                return iterator.hasNext();
            }

            @Override
            public URL nextElement() {
                return iterator.next();
            }
        };
    }

    private Class<?> processLoadedClass(Class<?> loadedClass) {
        final PluginConfig pluginConfig = loadedClass.getAnnotation(PluginConfig.class);
        if (pluginConfig != null) {
            // Set up the plugin config when loaded by class loader at the first time.
            // Agent class loader just loaded limited classes in the plugin jar(s), so the cost of this
            // isAssignableFrom would be also very limited.
            // 如果被加载的类标注了@PluginConfig注解,会加载插件的配置
            SnifferConfigInitializer.initializeConfig(pluginConfig.root());
        }

        return loadedClass;
    }

    private List<Jar> getAllJars() {
        if (allJars == null) {
            jarScanLock.lock();
            try {
                if (allJars == null) {
                    allJars = doGetJars();
                }
            } finally {
                jarScanLock.unlock();
            }
        }

        return allJars;
    }

    private LinkedList<Jar> doGetJars() {
        LinkedList<Jar> jars = new LinkedList<>();
        for (File path : classpath) {
            if (path.exists() && path.isDirectory()) {
                String[] jarFileNames = path.list((dir, name) -> name.endsWith(".jar"));
                for (String fileName : jarFileNames) {
                    try {
                        File file = new File(path, fileName);
                        Jar jar = new Jar(new JarFile(file), file);
                        jars.add(jar);
                        LOGGER.info("{} loaded.", file.toString());
                    } catch (IOException e) {
                        LOGGER.error(e, "{} jar file can't be resolved", fileName);
                    }
                }
            }
        }
        return jars;
    }

    @RequiredArgsConstructor
    private static class Jar {
        private final JarFile jarFile;
        private final File sourceFile;
    }
}

除了并行类加载外，AgentClassLoader的几个关键点：

1. AgentClassLoader的父类加载器是AppClassLoader
2. AgentClassLoader的classpath默认是agent主目录下的plugins和activations目录
3. 如果被加载的类标注了@PluginConfig注解，会加载插件的配置

小结：

4、插件定义体系

以dubbo 2.7.x的插件为例，我们来看下插件定义体系

1）、插件的定义

/**
 * 插件的定义,继承xxxPluginDefine,通常命名为xxxInstrumentation
 */
public class DubboInstrumentation extends ClassInstanceMethodsEnhancePluginDefine {

    private static final String ENHANCE_CLASS = "org.apache.dubbo.monitor.support.MonitorFilter";

    private static final String INTERCEPT_CLASS = "org.apache.skywalking.apm.plugin.asf.dubbo.DubboInterceptor";

    /**
     * 指定插件增强哪个类的字节码
     */
    @Override
    protected ClassMatch enhanceClass() {
        return NameMatch.byName(ENHANCE_CLASS);
    }

    /**
     * 拿到构造方法的拦截点
     */
    @Override
    public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
        return null;
    }

    /**
     * 拿到实例方法的拦截点
     */
    @Override
    public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
        return new InstanceMethodsInterceptPoint[] {
            new InstanceMethodsInterceptPoint() {
                /**
                 * 该插件增强的是MonitorFilter的invoke()方法
                 */
                @Override
                public ElementMatcher<MethodDescription> getMethodsMatcher() {
                    return named("invoke");
                }

                /**
                 * 交给DubboInterceptor进行字节码增强
                 */
                @Override
                public String getMethodsInterceptor() {
                    return INTERCEPT_CLASS;
                }

                /**
                 * 在字节码增强的过程中,是否要对原方法的入参进行改变
                 */
                @Override
                public boolean isOverrideArgs() {
                    return false;
                }
            }
        };
    }
}

拦截实例方法/构造器时，需要继承ClassInstanceMethodsEnhancePluginDefine

public abstract class ClassInstanceMethodsEnhancePluginDefine extends ClassEnhancePluginDefine {

    /**
     * @return null, means enhance no static methods.
     */
    @Override
    public StaticMethodsInterceptPoint[] getStaticMethodsInterceptPoints() {
        return null;
    }

}

拦截静态方法时，需要继承ClassStaticMethodsEnhancePluginDefine

public abstract class ClassStaticMethodsEnhancePluginDefine extends ClassEnhancePluginDefine {

    /**
     * @return null, means enhance no constructors.
     */
    @Override
    public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
        return null;
    }

    /**
     * @return null, means enhance no instance methods.
     */
    @Override
    public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
        return null;
    }
}

ClassInstanceMethodsEnhancePluginDefine和ClassStaticMethodsEnhancePluginDefine继承关系如下图：

AbstractClassEnhancePluginDefine是所有插件定义的顶级父类

2）、目标类匹配

ClassMatch是一个标志接口，表示类的匹配器

public interface ClassMatch {
}

NameMatch是通过完整类名精确匹配对应类

/**
 * Match the class with an explicit class name.
 * 通过完整类名精确匹配对应类
 */
public class NameMatch implements ClassMatch {
    private String className;

    private NameMatch(String className) {
        this.className = className;
    }

    public String getClassName() {
        return className;
    }

    public static NameMatch byName(String className) {
        return new NameMatch(className);
    }
}

除了NameMatch之外，还有比较常用的IndirectMatch用于间接匹配

public interface IndirectMatch extends ClassMatch {
    ElementMatcher.Junction buildJunction();

    // TypeDescription就是对类的描述,可以当做Class
    boolean isMatch(TypeDescription typeDescription);
}

IndirectMatch的子类，比如：PrefixMatch通过前缀匹配

public class PrefixMatch implements IndirectMatch {
    private String[] prefixes;

    private PrefixMatch(String... prefixes) {
        if (prefixes == null || prefixes.length == 0) {
            throw new IllegalArgumentException("prefixes argument is null or empty");
        }
        this.prefixes = prefixes;
    }

    @Override
    public ElementMatcher.Junction buildJunction() {
        ElementMatcher.Junction junction = null;

        // 拼接匹配条件
        // 有多个前缀,只要一个匹配即可
        for (String prefix : prefixes) {
            if (junction == null) {
                junction = ElementMatchers.nameStartsWith(prefix);
            } else {
                junction = junction.or(ElementMatchers.nameStartsWith(prefix));
            }
        }

        return junction;
    }

    @Override
    public boolean isMatch(TypeDescription typeDescription) {
        for (final String prefix : prefixes) {
            // typeDescription.getName()就是全类名
            if (typeDescription.getName().startsWith(prefix)) {
                return true;
            }
        }
        return false;
    }

    public static PrefixMatch nameStartsWith(final String... prefixes) {
        return new PrefixMatch(prefixes);
    }
}

3）、拦截器定义

public class DubboInterceptor implements InstanceMethodsAroundInterceptor {

DubboInterceptor继承了InstanceMethodsAroundInterceptor，InstanceMethodsAroundInterceptor源码如下：

public interface InstanceMethodsAroundInterceptor {
    /**
     * called before target method invocation.
     *
     * @param result change this result, if you want to truncate the method.
     */
    void beforeMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
        MethodInterceptResult result) throws Throwable;

    /**
     * called after target method invocation. Even method's invocation triggers an exception.
     *
     * @param ret the method's original return value. May be null if the method triggers an exception.
     * @return the method's actual return value.
     */
    Object afterMethod(EnhancedInstance objInst, Method method, Object[] allArguments, Class<?>[] argumentsTypes,
        Object ret) throws Throwable;

    /**
     * called when occur exception.
     *
     * @param t the exception occur.
     */
    void handleMethodException(EnhancedInstance objInst, Method method, Object[] allArguments,
        Class<?>[] argumentsTypes, Throwable t);
}

类似于AOP的环绕增强，在方法执行前、方法执行后、方法抛出异常时进行代码增强

4）、插件的声明

resources目录下的skywalking-plugin.def文件是对插件的声明

# 插件名=插件定义全类名
dubbo=org.apache.skywalking.apm.plugin.asf.dubbo.DubboInstrumentation

小结：

5、插件加载

public class SkyWalkingAgent {

    public static void premain(String agentArgs, Instrumentation instrumentation) throws PluginException {
        final PluginFinder pluginFinder;
        try {
            // 初始化配置
            SnifferConfigInitializer.initializeCoreConfig(agentArgs);
        } catch (Exception e) {
            // try to resolve a new logger, and use the new logger to write the error log here
            LogManager.getLogger(SkyWalkingAgent.class)
                    .error(e, "SkyWalking agent initialized failure. Shutting down.");
            return;
        } finally {
            // refresh logger again after initialization finishes
            LOGGER = LogManager.getLogger(SkyWalkingAgent.class);
        }

        try {
            // 加载插件
            pluginFinder = new PluginFinder(new PluginBootstrap().loadPlugins());
        } catch (AgentPackageNotFoundException ape) {
            LOGGER.error(ape, "Locate agent.jar failure. Shutting down.");
            return;
        } catch (Exception e) {
            LOGGER.error(e, "SkyWalking agent initialized failure. Shutting down.");
            return;
        }
      	// ...

premain()中调用PluginBootstrap的loadPlugins()方法实例化所有插件，然后将返回值用于构造PluginFinder对象

1）、PluginBootstrap实例化所有插件

public class PluginBootstrap {
    private static final ILog LOGGER = LogManager.getLogger(PluginBootstrap.class);

    /**
     * load all plugins.
     *
     * @return plugin definition list.
     */
    public List<AbstractClassEnhancePluginDefine> loadPlugins() throws AgentPackageNotFoundException {
        // 初始化自定义类加载器AgentClassLoader
        AgentClassLoader.initDefaultLoader();

        PluginResourcesResolver resolver = new PluginResourcesResolver();
        // 1)拿到所有skywalking-plugin.def的资源
        List<URL> resources = resolver.getResources();

        if (resources == null || resources.size() == 0) {
            LOGGER.info("no plugin files (skywalking-plugin.def) found, continue to start application.");
            return new ArrayList<AbstractClassEnhancePluginDefine>();
        }

      	// 2)
        for (URL pluginUrl : resources) {
            try {
                PluginCfg.INSTANCE.load(pluginUrl.openStream());
            } catch (Throwable t) {
                LOGGER.error(t, "plugin file [{}] init failure.", pluginUrl);
            }
        }

        List<PluginDefine> pluginClassList = PluginCfg.INSTANCE.getPluginClassList();

        List<AbstractClassEnhancePluginDefine> plugins = new ArrayList<AbstractClassEnhancePluginDefine>();
        for (PluginDefine pluginDefine : pluginClassList) {
            try {
                LOGGER.debug("loading plugin class {}.", pluginDefine.getDefineClass());
                // 3)通过AgentClassLoader实例化插件定义类实例
                AbstractClassEnhancePluginDefine plugin = (AbstractClassEnhancePluginDefine) Class.forName(pluginDefine.getDefineClass(), true, AgentClassLoader
                    .getDefault()).newInstance();
                plugins.add(plugin);
            } catch (Throwable t) {
                LOGGER.error(t, "load plugin [{}] failure.", pluginDefine.getDefineClass());
            }
        }

        // 4)加载基于xml定义的插件
        plugins.addAll(DynamicPluginLoader.INSTANCE.load(AgentClassLoader.getDefault()));

        return plugins;

    }

1. 代码1)处调用PluginResourcesResolver的getResources()方法
2. 代码2)处遍历skywalking-plugin.def的资源，调用PluginCfg的load()方法
3. 代码3)处通过AgentClassLoader实例化插件定义类实例
4. 由于SkyWalking Agent支持通过xml定义插件，代码4)处会加载基于xml定义的插件

先来看下PluginResourcesResolver的getResources()方法：

public class PluginResourcesResolver {
    private static final ILog LOGGER = LogManager.getLogger(PluginResourcesResolver.class);

    public List<URL> getResources() {
        List<URL> cfgUrlPaths = new ArrayList<URL>();
        Enumeration<URL> urls;
        try {
            urls = AgentClassLoader.getDefault().getResources("skywalking-plugin.def");

            while (urls.hasMoreElements()) {
                URL pluginUrl = urls.nextElement();
                cfgUrlPaths.add(pluginUrl);
                LOGGER.info("find skywalking plugin define in {}", pluginUrl);
            }

            return cfgUrlPaths;
        } catch (IOException e) {
            LOGGER.error("read resources failure.", e);
        }
        return null;
    }

这里实际上就是使用AgentClassLoader拿到所有skywalking-plugin.def的资源

PluginCfg的load()方法源码如下：

public enum PluginCfg {
    INSTANCE;

    private static final ILog LOGGER = LogManager.getLogger(PluginCfg.class);

    private List<PluginDefine> pluginClassList = new ArrayList<PluginDefine>();
    private PluginSelector pluginSelector = new PluginSelector();

    void load(InputStream input) throws IOException {
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(input));
            String pluginDefine;
            while ((pluginDefine = reader.readLine()) != null) {
                try {
                    if (pluginDefine.trim().length() == 0 || pluginDefine.startsWith("#")) {
                        continue;
                    }
                    // skywalking-plugin.def转换为PluginDefine对象
                    PluginDefine plugin = PluginDefine.build(pluginDefine);
                    pluginClassList.add(plugin);
                } catch (IllegalPluginDefineException e) {
                    LOGGER.error(e, "Failed to format plugin({}) define.", pluginDefine);
                }
            }
            // 剔除配置文件中指定的不需要启用的插件
            pluginClassList = pluginSelector.select(pluginClassList);
        } finally {
            input.close();
        }
    }

    public List<PluginDefine> getPluginClassList() {
        return pluginClassList;
    }

}

PluginCfg的load()方法会把skywalking-plugin.def转换为PluginDefine对象（属性：插件名和插件定义的类名），最后剔除配置文件中指定的不需要启用的插件

public class PluginDefine {
    /**
     * Plugin name.
     */
    private String name;

    /**
     * The class name of plugin defined.
     */
    private String defineClass;

2）、PluginFinder分类插件

public class PluginFinder {
    /**
     * 为什么这里的Map泛型是<String,LinkedList>
     * 因为对于同一个类,可能有多个插件都要对它进行字节码增强
     * key => 目标类
     * value => 所有可以对这个目标类生效的插件
     */
    private final Map<String, LinkedList<AbstractClassEnhancePluginDefine>> nameMatchDefine = new HashMap<String, LinkedList<AbstractClassEnhancePluginDefine>>();
    private final List<AbstractClassEnhancePluginDefine> signatureMatchDefine = new ArrayList<AbstractClassEnhancePluginDefine>();
    private final List<AbstractClassEnhancePluginDefine> bootstrapClassMatchDefine = new ArrayList<AbstractClassEnhancePluginDefine>();

    public PluginFinder(List<AbstractClassEnhancePluginDefine> plugins) {
        // 对加载的所有插件做分类
        for (AbstractClassEnhancePluginDefine plugin : plugins) {
            ClassMatch match = plugin.enhanceClass();

            if (match == null) {
                continue;
            }

            if (match instanceof NameMatch) {
                NameMatch nameMatch = (NameMatch) match;
                LinkedList<AbstractClassEnhancePluginDefine> pluginDefines = nameMatchDefine.get(nameMatch.getClassName());
                if (pluginDefines == null) {
                    pluginDefines = new LinkedList<AbstractClassEnhancePluginDefine>();
                    nameMatchDefine.put(nameMatch.getClassName(), pluginDefines);
                }
                pluginDefines.add(plugin);
            } else {
                signatureMatchDefine.add(plugin);
            }

            if (plugin.isBootstrapInstrumentation()) {
                bootstrapClassMatchDefine.add(plugin);
            }
        }
    }

    /**
     * 查找所有能够对指定类型生效的插件
     * 1.从命名插件里找
     * 2.从间接匹配插件里找
     *
     * @param typeDescription 可以看做是class
     * @return
     */
    public List<AbstractClassEnhancePluginDefine> find(TypeDescription typeDescription) {
        List<AbstractClassEnhancePluginDefine> matchedPlugins = new LinkedList<AbstractClassEnhancePluginDefine>();
        String typeName = typeDescription.getTypeName();
        if (nameMatchDefine.containsKey(typeName)) {
            matchedPlugins.addAll(nameMatchDefine.get(typeName));
        }

        for (AbstractClassEnhancePluginDefine pluginDefine : signatureMatchDefine) {
            IndirectMatch match = (IndirectMatch) pluginDefine.enhanceClass();
            if (match.isMatch(typeDescription)) {
                matchedPlugins.add(pluginDefine);
            }
        }

        return matchedPlugins;
    }

    /**
     * 将所有插件中匹配类的逻辑做一个聚合
     *
     * @return 整个Agent需要拦截的类的所有条件的集合
     */
    public ElementMatcher<? super TypeDescription> buildMatch() {
        ElementMatcher.Junction judge = new AbstractJunction<NamedElement>() {
            @Override
            public boolean matches(NamedElement target) {
                return nameMatchDefine.containsKey(target.getActualName());
            }
        };
        judge = judge.and(not(isInterface()));
        for (AbstractClassEnhancePluginDefine define : signatureMatchDefine) {
            ClassMatch match = define.enhanceClass();
            if (match instanceof IndirectMatch) {
                judge = judge.or(((IndirectMatch) match).buildJunction());
            }
        }
        return new ProtectiveShieldMatcher(judge);
    }

小结：

参考：

SkyWalking8.7.0源码分析（如果你对SkyWalking Agent源码感兴趣的话，强烈建议看下该教程）