Skywalking 从架构上分为三个大模块，分别是Agent、Oap（Backend&Storage）、UI。这篇文档主要是介绍Agent部分。这部分核心依赖的是java的agent技术，对这个技术陌生的可以先看一下Agent 这篇文章。Skywalking在agent的基础上利用ByteBuddy类库对类和方法等进行加强，实现无入侵的监测。
SkyWalkingAgent类
在官方的skywalking-agent包中，通过MANIFEST.MF的Premain-Class值，我们可以确定入口类是org.apache.skywalking.apm.agent.SkyWalkingAgent，而核心的方法就是里面的premain方法，大概的内容是以下四部分。
读取并整合配置
加载插件
通过AgentBuilder注入到jvm
启动上报相关的服务等
读取并整合配置
//将各个途径的配置都处理汇总到CONFIG类里，CONFIG类里面全是静态字段，所以直接赋值也不需要实例化对象
SnifferConfigInitializer.initializeCoreConfig(agentArgs);
public static void initializeCoreConfig(String agentOptions) {
AGENT_SETTINGS = new Properties();
//loadConfig方法中会优先从指定的系统属性skywalking_config中取值（即 java -Dskywalking_config=xxx/custom.config)
//如果没有设置，则默认加载SKYWALKING_AGENT_PATH/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);
//替换占位符的变量，依次从System.getProperty(key)->System.getenv(key)->properties.getProperty(key)->default取
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 {
//替换通过java -Dskywalking.指定的变量值
//这里我觉得很奇怪，明明上面也是处理系统属性，这里为什么还要专门用skywalking前缀区分呢？
overrideConfigBySystemProp();
} catch (Exception e) {
LOGGER.error(e, "Failed to read the system properties.");
}
agentOptions = StringUtil.trim(agentOptions, ',');
if (!StringUtil.isEmpty(agentOptions)) {
try {
agentOptions = agentOptions.trim();
LOGGER.info("Agent options is {}.", agentOptions);
//替换在 java -agent:/***.jar=key:value里的值
overrideConfigByAgentOptions(agentOptions);
} catch (Exception e) {
LOGGER.error(e, "Failed to parse the agent options, val is {}.", agentOptions);
}
}
//通过反射将AGENT_SETTINGS注入到Config的变量中
initializeConfig(Config.class);
...
}
加载插件
//这一步实际上是加载了插件jar包里的增加类def里面描述的类，最终都传到了PluginFinder，存储为三种不同的对象
pluginFinder = new PluginFinder(new PluginBootstrap().loadPlugins());
public List<AbstractClassEnhancePluginDefine> loadPlugins() throws AgentPackageNotFoundException {
//可以通过启动一次看日志，了解这里是先装载jar包再识别出def
//注意，这里自定义了一个类加载器,并且加载了activations和plugins/plugins下的jar包
//Q&A 2023/8/27
// Q:为什么要自定义类加载器
// A:为了加载插件的那些jar包，继承自AppClassLodder
AgentClassLoader.initDefaultLoader();
PluginResourcesResolver resolver = new PluginResourcesResolver();
//获取所有的skywalking-plugin.def，由于上面的jar包已经添加，所以实际就是那些jar包里的skywalking-plugin.def
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>();
}
for (URL pluginUrl : resources) {
try {
//最终都加载到了List<PluginDefine> pluginClassList里
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());
//Q&A 2023/8/22
// Q:为什么这里所有的类都能强转为AbstractClassEnhancePluginDefine
// A:因为插件继承的顶层最终都是这个抽象类,见下面类图
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());
}
}
//这个是通过SPI用所有的InstrumentationLoader来添加AbstractClassEnhancePluginDefine，不知道用在哪
plugins.addAll(DynamicPluginLoader.INSTANCE.load(AgentClassLoader.getDefault()));
return plugins;
}
//这里面将插件分到了三个不同的集合中
public PluginFinder(List<AbstractClassEnhancePluginDefine> plugins) {
for (AbstractClassEnhancePluginDefine plugin : plugins) {
ClassMatch match = plugin.enhanceClass();
if (match == null) {
continue;
}
//重写enhanceClass，以restTemplate为例，enhanceClass是NameMatch.byName("org.springframework.web.client.RestTemplate")，所以对应map的key就是"org.springframework.web.client.RestTemplate"
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);
}
}
}
AgentBuilder
这中间有一部分是JDK9的模块相关的内容，由于我还没有深入了解过JDK9相关的内容，加上
//这一步实际上是加载了插件jar包里的增加类def里面描述的类，最终都传到了PluginFinder，存储为三种不同的对象
pluginFinder = new PluginFinder(new PluginBootstrap().loadPlugins());
public List<AbstractClassEnhancePluginDefine> loadPlugins() throws AgentPackageNotFoundException {
//可以通过启动一次看日志，了解这里是先装载jar包再识别出def
//注意，这里自定义了一个类加载器,并且加载了activations和plugins/plugins下的jar包
//Q&A 2023/8/27
// Q:为什么要自定义类加载器
// A:为了加载插件的那些jar包，继承自AppClassLodder
AgentClassLoader.initDefaultLoader();
PluginResourcesResolver resolver = new PluginResourcesResolver();
//获取所有的skywalking-plugin.def，由于上面的jar包已经添加，所以实际就是那些jar包里的skywalking-plugin.def
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>();
}
for (URL pluginUrl : resources) {
try {
//最终都加载到了List<PluginDefine> pluginClassList里
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());
//Q&A 2023/8/22
// Q:为什么这里所有的类都能强转为AbstractClassEnhancePluginDefine
// A:因为插件继承的顶层最终都是这个抽象类,见下面类图
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());
}
}
//这个是通过SPI用所有的InstrumentationLoader来添加AbstractClassEnhancePluginDefine，不知道用在哪
plugins.addAll(DynamicPluginLoader.INSTANCE.load(AgentClassLoader.getDefault()));
return plugins;
}
//这里面将插件分到了三个不同的集合中
public PluginFinder(List<AbstractClassEnhancePluginDefine> plugins) {
for (AbstractClassEnhancePluginDefine plugin : plugins) {
ClassMatch match = plugin.enhanceClass();
if (match == null) {
continue;
}
//重写enhanceClass，以restTemplate为例，enhanceClass是NameMatch.byName("org.springframework.web.client.RestTemplate")，所以对应map的key就是"org.springframework.web.client.RestTemplate"
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);
}
}
}
这里开始就要用到ByteBuddy了，先简单了解下AgentBuilder ，然后我们结合插件分析源码中的核心部分
比较关键的其实就是type、transform和installOn三个方法，其他的ignore、with感兴趣的可以自己研究，不影响主流程的理解
type()
type是命中要处理的类，对应的是插件里的enhanceClass，如果enhanceClass实现是指定的类名称，那么在之前已经加入到nameMatchDefine里了，如果不是则各自解析，比如Spring的插件就是通过注解来匹配
protected ClassMatch enhanceClass() {
return ClassAnnotationMatch.byClassAnnotationMatch(this.getEnhanceAnnotations());
}
protected String[] getEnhanceAnnotations() {
return new String[]{"org.springframework.web.bind.annotation.RestController"};
}
/**
* 主要是通过插件里定义的NameMatch和IndirectMatch来匹配，IndirectMatch就是要更复杂，比如匹配的更多，前缀，后缀之类的，参考IndirectMatch的实现类
* @return
*/
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);
transform()
public DynamicType.Builder<?> define(TypeDescription typeDescription, DynamicType.Builder<?> builder,
...
WitnessFinder finder = WitnessFinder.INSTANCE;
//witness机制，希望加强某类之前，必须有些类已经加载，用于区分版本，让插件和应用的版本所对应
/**
* find witness classes for enhance class
*/
String[] witnessClasses = witnessClasses();
if (witnessClasses != null) {
for (String witnessClass : witnessClasses) {
if (!finder.exist(witnessClass, classLoader)) {
LOGGER.warn("enhance class {} by plugin {} is not activated. Witness class {} does not exist.", transformClassName, interceptorDefineClassName, witnessClass);
return null;
}
}
}
List<WitnessMethod> witnessMethods = witnessMethods();
if (!CollectionUtil.isEmpty(witnessMethods)) {
for (WitnessMethod witnessMethod : witnessMethods) {
if (!finder.exist(witnessMethod, classLoader)) {
LOGGER.warn("enhance class {} by plugin {} is not activated. Witness method {} does not exist.", transformClassName, interceptorDefineClassName, witnessMethod);
return null;
}
}
}
/**
* find origin class source code for interceptor
*/
DynamicType.Builder<?> newClassBuilder = this.enhance(typeDescription, builder, classLoader, context);
...
}
transform是增强命中的类，这里核心方法是org.apache.skywalking.apm.agent.SkyWalkingAgent.Transformer#transform->
org.apache.skywalking.apm.agent.core.plugin.AbstractClassEnhancePluginDefine#define
Witness机制
这里需要提及一个机制，即Witness机制，他的作用是让引入的插件必须和当前应用组件的版本必须相对应。举例来说，spring-mvc有三个版本的插件，如下面代码所示，不同的版本校验的类并不相同。(但是我去查了spring3456每个版本的类，好像并不太能对应到spring的版本，所以我也不知道skywalking官方的这个mvc插件对应的哪个版本)
public abstract class AbstractSpring3Instrumentation extends ClassInstanceMethodsEnhancePluginDefine {
public static final String WITHNESS_CLASSES = "org.springframework.web.servlet.view.xslt.AbstractXsltView";
public AbstractSpring3Instrumentation() {
}
protected final String[] witnessClasses() {
return new String[]{"org.springframework.web.servlet.view.xslt.AbstractXsltView"};
}
}
public abstract class AbstractSpring4Instrumentation extends ClassInstanceMethodsEnhancePluginDefine {
public static final String WITHNESS_CLASSES = "org.springframework.cache.interceptor.SimpleKey";
public AbstractSpring4Instrumentation() {
}
protected String[] witnessClasses() {
return new String[]{"org.springframework.cache.interceptor.SimpleKey", "org.springframework.cache.interceptor.DefaultKeyGenerator"};
}
}
public abstract class AbstractSpring5Instrumentation extends ClassInstanceMethodsEnhancePluginDefine {
public static final String WITNESS_CLASSES = "org.springframework.beans.annotation.AnnotationBeanUtils";
public AbstractSpring5Instrumentation() {
}
protected final String[] witnessClasses() {
return new String[]{"org.springframework.beans.annotation.AnnotationBeanUtils"};
}
}
拦截器
enhance方法里面包含了两个部分，enhanceClass主要是加强类的静态方法，enhanceInstance则是加强类的构造方法和普通方法.
这样给插件留出更细粒度的扩展，只需要插件实现InstanceMethodsInterceptPoint等接口就可以了
protected DynamicType.Builder<?> enhance(TypeDescription typeDescription, DynamicType.Builder<?> newClassBuilder,
ClassLoader classLoader, EnhanceContext context) throws PluginException {
//这里是实际和插件作用的地方
newClassBuilder = this.enhanceClass(typeDescription, newClassBuilder, classLoader);
newClassBuilder = this.enhanceInstance(typeDescription, newClassBuilder, classLoader, context);
return newClassBuilder;
}
protected DynamicType.Builder<?> enhanceInstance(TypeDescription typeDescription,
DynamicType.Builder<?> newClassBuilder, ClassLoader classLoader,
EnhanceContext context) throws PluginException {
...
/**
* Manipulate class source code.<br/>
*
* new class need:<br/>
* 1.Add field, name {@link #CONTEXT_ATTR_NAME}.
* 2.Add a field accessor for this field.
*
* And make sure the source codes manipulation only occurs once.
*
*/
//生成一个新的类，该类继承EnhancedInstance接口，并且自定义了一个字段叫做_$EnhancedClassField_ws，同时生成了这个的set、get方法
//Q&A 2023/8/29
// Q: 为什么非要指定这样一个字段呢？
// A: 说是为了解决上下文问题，需要这样一个字段缓存，但是我不理解 https://github.com/apache/skywalking/issues/7146
if (!typeDescription.isAssignableTo(EnhancedInstance.class)) {
if (!context.isObjectExtended()) {
newClassBuilder = newClassBuilder.defineField(
CONTEXT_ATTR_NAME, Object.class, ACC_PRIVATE | ACC_VOLATILE)
.implement(EnhancedInstance.class)
.intercept(FieldAccessor.ofField(CONTEXT_ATTR_NAME));
context.extendObjectCompleted();
}
}
//下面的增强构造和实例方法其实很类似
//核心点在于InstanceMethodsInterceptPoint，
//通过getMethodsMatcher获取需要加强的方法
//通过getMethodsInterceptor获取需要插件定义的拦截器，然后将其构造到agentBuilder里
/**
* 2. enhance constructors
*/
...
/**
* 3. enhance instance methods
*/
...
newClassBuilder = newClassBuilder.method(junction)
.intercept(MethodDelegation.withDefaultConfiguration()
.to(new InstMethodsInter(interceptor, classLoader)));
}
return newClassBuilder;
}
这里不对agentBuidle的intercept等具体分析，感兴趣的可以自己看
installOn()
正常定义permain方法时，定义的transformer都是通过addTransformer添加到instrumentation的
public static void premain(String agentArgs, Instrumentation instrumentation) {
System.out.println("enhance by agent,params:"+agentArgs);
instrumentation.addTransformer(new ClassFileTransformer() {
@Override
public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
ProtectionDomain protectionDomain, byte[] classfileBuffer)
throws IllegalClassFormatException {
System.out.println("premain load Class :" + className);
return classfileBuffer;
}
}, true);
}
而这里，我们用的是agentBuilder,在transform()部分我们已经知道定义了transformer,其transform方法是遍历插件定义然后加强类，因此我们可以猜测最终肯定也是将transformer添加到instrumentation，最简单的映证方法就是在sun.instrument.InstrumentationImpl#addTransformer(java.lang.instrument.ClassFileTransformer, boolean)打个断点，启动项目
installon()->org.apache.skywalking.apm.dependencies.net.bytebuddy.agent.builder.AgentBuilder.Default#doInstall()
private ResettableClassFileTransformer doInstall(Instrumentation instrumentation, AgentBuilder.RawMatcher matcher, AgentBuilder.PatchMode.Handler handler) {
if (!this.circularityLock.acquire()) {
throw new IllegalStateException("Could not acquire the circularity lock upon installation.");
} else {
ResettableClassFileTransformer var13;
try {
AgentBuilder.RedefinitionStrategy.ResubmissionStrategy.Installation installation = this.redefinitionResubmissionStrategy.apply(instrumentation, this.poolStrategy, this.locationStrategy, this.descriptionStrategy, this.fallbackStrategy, this.listener, this.installationListener, this.circularityLock, new AgentBuilder.Default.Transformation.SimpleMatcher(this.ignoreMatcher, this.transformations), this.redefinitionStrategy, this.redefinitionBatchAllocator, this.redefinitionListener);
ResettableClassFileTransformer classFileTransformer = this.transformerDecorator.decorate(this.makeRaw(installation.getListener(), installation.getInstallationListener(), installation.getResubmissionEnforcer()));
installation.getInstallationListener().onBeforeInstall(instrumentation, classFileTransformer);
try {
this.warmupStrategy.apply(classFileTransformer, this.locationStrategy, this.redefinitionStrategy, this.circularityLock, installation.getInstallationListener());
handler.onBeforeRegistration(instrumentation);
if (this.redefinitionStrategy.isRetransforming()) {
DISPATCHER.addTransformer(instrumentation, classFileTransformer, true);
} else {
instrumentation.addTransformer(classFileTransformer);
}
启动上报相关的服务
暂未研究