了解skywalking的agent

Author: qisiii

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

@@ -73,6 +73,7 @@ public static void initializeCoreConfig(String agentOptions) {
             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
                 AGENT_SETTINGS.put(key, PropertyPlaceholderHelper.INSTANCE.replacePlaceholders(value, AGENT_SETTINGS));
             }
 
@@ -81,6 +82,7 @@ public static void initializeCoreConfig(String agentOptions) {
         }
 
         try {
+            //替换通过java -Dskywalking.指定的变量值
             overrideConfigBySystemProp();
         } catch (Exception e) {
             LOGGER.error(e, "Failed to read the system properties.");
@@ -91,7 +93,7 @@ public static void initializeCoreConfig(String 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);
@@ -243,6 +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
         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/plugin/PluginBootstrap.java

@@ -39,9 +39,12 @@ public class PluginBootstrap {
      * @return plugin definition list.
      */
     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
         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) {

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

@@ -52,6 +52,7 @@ void load(InputStream input) throws IOException {
                     LOGGER.error(e, "Failed to format plugin({}) define.", pluginDefine);
                 }
             }
+            //可以排除插件
             pluginClassList = pluginSelector.select(pluginClassList);
         } finally {
             input.close();

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

@@ -86,6 +86,7 @@ public AgentClassLoader(ClassLoader parent) throws AgentPackageNotFoundException
         super(parent);
         File agentDictionary = AgentPackagePath.getPath();
         classpath = new LinkedList<>();
+        //将jar包加到类路径
         Config.Plugin.MOUNT.forEach(mountFolder -> classpath.add(new File(agentDictionary, mountFolder)));
     }
 

apm-sniffer/apm-agent/pom.xml

@@ -68,6 +68,7 @@
                             <transformers>
                                 <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                     <manifestEntries>
+<!--                                        在这里指定了premain-class-->
                                         <Premain-Class>${premain.class}</Premain-Class>
                                         <Can-Redefine-Classes>${can.redefine.classes}</Can-Redefine-Classes>
                                         <Can-Retransform-Classes>${can.retransform.classes}</Can-Retransform-Classes>

apm-sniffer/apm-agent/src/main/java/org/apache/skywalking/apm/agent/SkyWalkingAgent.java

@@ -90,7 +90,7 @@ public static void premain(String agentArgs, Instrumentation instrumentation) th
             LOGGER.error(e, "SkyWalking agent initialized failure. Shutting down.");
             return;
         }
-
+        //byte-buddy是一个字节码生成和操作库
         final ByteBuddy byteBuddy = new ByteBuddy().with(TypeValidation.of(Config.Agent.IS_OPEN_DEBUGGING_CLASS));
 
         AgentBuilder agentBuilder = new AgentBuilder.Default(byteBuddy).ignore(