研究启动和上报逻辑

Author: qisiii

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/conf/SnifferConfigInitializer.java

@@ -245,7 +245,7 @@ private static void setAgentVersion() {
      * @return the config file {@link InputStream}, or null if not needEnhance.
      */
     private static InputStreamReader loadConfig() throws AgentPackageNotFoundException, ConfigNotFoundException {
-        //从系统属性里取自定义路径或者从默认路径/config/agent.config初始化配置
+        //从系统属性（即 java -Dskywalking_config=xxx/custom.config）里取自定义配置或者从默认路径/config/agent.config初始化配置
         String specifiedConfigPath = System.getProperty(SPECIFIED_CONFIG_PATH);
         File configFile = StringUtil.isEmpty(specifiedConfigPath) ? new File(
             AgentPackagePath.getPath(), DEFAULT_CONFIG_FILE_NAME) : new File(specifiedConfigPath);

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/trace/AbstractTracingSpan.java

@@ -145,6 +145,7 @@ public AbstractTracingSpan tag(AbstractTag<?> tag, String value) {
      */
     public boolean finish(TraceSegment owner) {
         this.endTime = System.currentTimeMillis();
+        //在这里进行的封装和消费
         owner.archive(this);
         return true;
     }

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/trace/TraceSegment.java

@@ -140,7 +140,7 @@ public void setIgnore(boolean ignore) {
 
     /**
      * This is a high CPU cost method, only called when sending to collector or test cases.
-     *
+     * 在这里对span进行封装，通过消费者线程池进行消费发送到oap
      * @return the segment as GRPC service parameter
      */
     public SegmentObject transform() {

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/plugin/AbstractClassEnhancePluginDefine.java

@@ -68,7 +68,9 @@ public DynamicType.Builder<?> define(TypeDescription typeDescription, DynamicTyp
 
         LOGGER.debug("prepare to enhance class {} by {}.", transformClassName, interceptorDefineClassName);
         WitnessFinder finder = WitnessFinder.INSTANCE;
-        //witness感觉更像是在加载某类之前，必须有些类已经加载
+        //witness机制
+        // 如果必须在特定的版本中激活工具，则设置 witessClass 和/或 witessMethod。
+        // 必须已经存在指定类或者方法之后，才能让某些插件起效
         /**
          * find witness classes for enhance class
          */

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/plugin/PluginBootstrap.java

@@ -41,6 +41,9 @@ public class PluginBootstrap {
     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/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();

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/plugin/interceptor/enhance/ClassEnhancePluginDefine.java

@@ -96,6 +96,10 @@ protected DynamicType.Builder<?> enhanceInstance(TypeDescription typeDescription
          * 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(
@@ -105,13 +109,18 @@ protected DynamicType.Builder<?> enhanceInstance(TypeDescription typeDescription
                 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
          */
         if (existedConstructorInterceptPoint) {
             for (ConstructorInterceptPoint constructorInterceptPoint : constructorInterceptPoints) {
                 if (isBootstrapInstrumentation()) {
+                    //看不懂具体的intercept的原理，但目前也不关心
                     newClassBuilder = newClassBuilder.constructor(constructorInterceptPoint.getConstructorMatcher())
                                                      .intercept(SuperMethodCall.INSTANCE.andThen(MethodDelegation.withDefaultConfiguration()
                                                                                                                  .to(BootstrapInstrumentBoost