skywalking源码流程

　　我们已经知道skywalking的入口就是premain方法：

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
            //异常重新获取日志类型
            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;
        }

        //创建bytebuddy
        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 {
            ServiceManager.INSTANCE.boot();
        } catch (Exception e) {
            LOGGER.error(e, "Skywalking agent boot failure.");
        }

        //添加shutdown
        Runtime.getRuntime()
                .addShutdownHook(new Thread(ServiceManager.INSTANCE::shutdown, "skywalking service shutdown thread"));
    }

可以看到首先执行的是加载配置的方法，我们往下看：

public static void initializeCoreConfig(String agentOptions) {
        AGENT_SETTINGS = new Properties();
        //读取指定文件中配置并放入AGENT_SETTINGS中
        try (final InputStreamReader configFileStream = loadConfig()) {
            AGENT_SETTINGS.load(configFileStream);
            for (String key : AGENT_SETTINGS.stringPropertyNames()) {
                String value = (String) AGENT_SETTINGS.get(key);
                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 {
            //读取系统环境变量配置
            overrideConfigBySystemProp();
        } catch (Exception e) {
            LOGGER.error(e, "Failed to read the system properties.");
        }

        //读取-javaagent传入的参数
        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);
            }
        }
        //将上面读取完的参数都放到Config配置里面
        initializeConfig(Config.class);
        // reconfigure logger after config initialization
        //设置日志级别
        configureLogger();
        LOGGER = LogManager.getLogger(SnifferConfigInitializer.class);

        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;
        }

        IS_INIT_COMPLETED = true;
    }

这里会从指定文件，环境变量和指定参数配置下依次读取，然后设置日志级别。

接下来就是插件的加载过程：

.tb_button { padding: 1px; cursor: pointer; border-right: 1px solid rgba(139, 139, 139, 1); border-left: 1px solid rgba(255, 255, 255, 1); border-bottom: 1px solid rgba(255, 255, 255, 1) }
.tb_button.hover { borer: 2px outset #def; background-color: rgba(248, 248, 248, 1) !important }
.ws_toolbar { z-index: 100000 }
.ws_toolbar .ws_tb_btn { cursor: pointer; border: 1px solid rgba(85, 85, 85, 1); padding: 3px }
.tb_highlight { background-color: rgba(255, 255, 0, 1) }
.tb_hide { visibility: hidden }
.ws_toolbar img { padding: 2px; margin: 0 }
.tb_button { padding: 1px; cursor: pointer; border-right: 1px solid rgba(139, 139, 139, 1); border-left: 1px solid rgba(255, 255, 255, 1); border-bottom: 1px solid rgba(255, 255, 255, 1) }
.tb_button.hover { borer: 2px outset #def; background-color: rgba(248, 248, 248, 1) !important }
.ws_toolbar { z-index: 100000 }
.ws_toolbar .ws_tb_btn { cursor: pointer; border: 1px solid rgba(85, 85, 85, 1); padding: 3px }
.tb_highlight { background-color: rgba(255, 255, 0, 1) }
.tb_hide { visibility: hidden }
.ws_toolbar img { padding: 2px; margin: 0 }
.tb_button { padding: 1px; cursor: pointer; border-right: 1px solid rgba(139, 139, 139, 1); border-left: 1px solid rgba(255, 255, 255, 1); border-bottom: 1px solid rgba(255, 255, 255, 1) }
.tb_button.hover { borer: 2px outset #def; background-color: rgba(248, 248, 248, 1) !important }
.ws_toolbar { z-index: 100000 }
.ws_toolbar .ws_tb_btn { cursor: pointer; border: 1px solid rgba(85, 85, 85, 1); padding: 3px }
.tb_highlight { background-color: rgba(255, 255, 0, 1) }
.tb_hide { visibility: hidden }
.ws_toolbar img { padding: 2px; margin: 0 }