MDC skywalking与ThreadLocal
MDC
A Mapped Diagnostic Context, or MDC in short, is an instrument for distinguishing interleaved log output from different sources. Log output is typically interleaved when a server handles multiple clients near-simultaneously.
The MDC is managed on a per thread basis. Note that a child thread does not inherit the mapped diagnostic context of its parent.
映射诊断上下文，简称MDC，是一种用于区分来自不同来源的交错日志输出的工具。当服务器几乎同时处理多个客户端时，日志输出通常是交错的。
MDC基于每个线程进行管理。请注意，子线程不继承其父线程的MDC。
以上解释来自LogbackMDCAdapter（pom定义如下）
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.4.7</version>
</dependency>
ThreadLocal的功能是为当前线程维护一个map
org.slf4j.MDC基于ThreadLocal，集成了日志组件，具体实现有Log4jMDCAdapter和LogbackMDCAdapter
org.slf4j.MDC的ThreadLocal里的key，可以在log4j/logback.xml中定义占位符，并输出到日志
比如，执行MDC.put("traceId", traceId)，logback.xml中定义如下，最后输出的日志，就能打印出traceId
<!--格式化输出：%d表示日期，%thread表示线程名，%traceId表示分布式链路id，%-5level：级别从左显示5个字符宽度，%msg表示日志消息，%n表示换行符-->
<pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] [%traceId] %-5level - %msg%n</pattern>
至于MDC中的ThreadLocal，有没有跨线程复制的能力（父线程的上下文复制到子线程），与MDCAdapter的版本有关。
在1.4.7版本的LogbackMDCAdapter，注释写的是Note that a child thread does not inherit the mapped diagnostic context of its parent. 子线程不继承父线程的MDC。
在1.2.9版本的LogbackMDCAdapter，注释写的是A child thread automatically inherits a copy of the mapped diagnostic context of its parent. 子线程自动继承父线程的MDC。
Java Instrument && Java Agent
Java Instrument
Java Instrument是JDK1.5提供的一个新特性，用于监测JVM进程，甚至可以修改类的实现。有了这样的功能，就可以更灵活的实现运行时虚拟机监控和Java类操作，这样的特性可以看做是一种虚拟机级别的AOP实现方式。
image.png
ClassFileTransformer是类文件的转换器
Instrumentation提供监测Java程序所需的服务
Java Agent
Java Agent是一种特殊的Java程序，可以看做是Java Instrumentation的客户端。普通Java程序通过main函数启动，Java Agent无法单独启动，必须依附于一个Java程序上，与他运行在同一个进程中，通过Java Instrumentation的客户端 API与虚拟机交互。
JVM会把Instrumentation的实例作为参数注入到Java Agent的启动方法上，因此要使用Instrumentation功能，必须通过Java Agent。
Java Agent有2个加载时机，一个是JVM启动前通过-javaagent参数静态加载执行，另一个是JVM启动后通过Java Tool API中的attach api动态加载执行。
skywalking
skywalking的主要作用是以javaAgent的形式（也可以有别的形式），收集应用程序的链路信息（请求入参、响应结果、耗时等等），上传到存储组件，并提供可视化展示。
源码仓库: https://github.com/apache/skywalking.git
分支: 6.x
Trace注解
方法上添加了@Trace注解，会执行TraceAnnotationMethodInterceptor拦截器，拦截器中会创建span
package org.apache.skywalking.apm.toolkit.activation.trace;
import net.bytebuddy.description.method.MethodDescription;
import net.bytebuddy.matcher.ElementMatcher;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.DeclaredInstanceMethodsInterceptPoint;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.InstanceMethodsInterceptPoint;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.ClassInstanceMethodsEnhancePluginDefine;
import org.apache.skywalking.apm.agent.core.plugin.match.ClassMatch;
import org.apache.skywalking.apm.agent.core.plugin.match.MethodAnnotationMatch;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.ConstructorInterceptPoint;
import static net.bytebuddy.matcher.ElementMatchers.isAnnotatedWith;
import static net.bytebuddy.matcher.ElementMatchers.named;
/**
* {@link TraceAnnotationActivation} enhance all method that annotated with <code>org.apache.skywalking.apm.toolkit.trace.annotation.Trace</code>
* by <code>TraceAnnotationMethodInterceptor</code>.
*
* @author zhangxin
*/
public class TraceAnnotationActivation extends ClassInstanceMethodsEnhancePluginDefine {
public static final String TRACE_ANNOTATION_METHOD_INTERCEPTOR = "org.apache.skywalking.apm.toolkit.activation.trace.TraceAnnotationMethodInterceptor";
public static final String TRACE_ANNOTATION = "org.apache.skywalking.apm.toolkit.trace.Trace";
@Override public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
return new ConstructorInterceptPoint[0];
}
@Override public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
return new InstanceMethodsInterceptPoint[] {
new DeclaredInstanceMethodsInterceptPoint() {
@Override public ElementMatcher<MethodDescription> getMethodsMatcher() {
return isAnnotatedWith(named(TRACE_ANNOTATION));
}
@Override public String getMethodsInterceptor() {
return TRACE_ANNOTATION_METHOD_INTERCEPTOR;
}
@Override public boolean isOverrideArgs() {
return false;
}
}
};
}
@Override protected ClassMatch enhanceClass() {
return MethodAnnotationMatch.byMethodAnnotationMatch(new String[] {TRACE_ANNOTATION});
}
}
TraceCrossThread注解
类上添加了@TraceCrossThread注解，并且存在名为call, run, get的方法，会执行CallableOrRunnableInvokeInterceptor拦截器，拦截器中会创建span
package org.apache.skywalking.apm.toolkit.activation.trace;
import net.bytebuddy.description.method.MethodDescription;
import net.bytebuddy.matcher.ElementMatcher;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.ConstructorInterceptPoint;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.InstanceMethodsInterceptPoint;
import org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.ClassInstanceMethodsEnhancePluginDefine;
import org.apache.skywalking.apm.agent.core.plugin.match.ClassMatch;
import static net.bytebuddy.matcher.ElementMatchers.*;
import static org.apache.skywalking.apm.agent.core.plugin.match.ClassAnnotationMatch.byClassAnnotationMatch;
/**
* {@link CallableOrRunnableActivation} presents that skywalking intercepts all Class with annotation
* "org.skywalking.apm.toolkit.trace.TraceCrossThread" and method named "call" or "run".
*
* @author carlvine500 lican
*/
public class CallableOrRunnableActivation extends ClassInstanceMethodsEnhancePluginDefine {
public static final String ANNOTATION_NAME = "org.apache.skywalking.apm.toolkit.trace.TraceCrossThread";
private static final String INIT_METHOD_INTERCEPTOR = "org.apache.skywalking.apm.toolkit.activation.trace.CallableOrRunnableConstructInterceptor";
private static final String CALL_METHOD_INTERCEPTOR = "org.apache.skywalking.apm.toolkit.activation.trace.CallableOrRunnableInvokeInterceptor";
private static final String CALL_METHOD_NAME = "call";
private static final String RUN_METHOD_NAME = "run";
private static final String GET_METHOD_NAME = "get";
@Override public ConstructorInterceptPoint[] getConstructorsInterceptPoints() {
return new ConstructorInterceptPoint[] {
new ConstructorInterceptPoint() {
@Override public ElementMatcher<MethodDescription> getConstructorMatcher() {
return any();
}
@Override public String getConstructorInterceptor() {
return INIT_METHOD_INTERCEPTOR;
}
}
};
}
@Override
public InstanceMethodsInterceptPoint[] getInstanceMethodsInterceptPoints() {
return new InstanceMethodsInterceptPoint[] {
new InstanceMethodsInterceptPoint() {
@Override public ElementMatcher<MethodDescription> getMethodsMatcher() {
return (named(CALL_METHOD_NAME).and(takesArguments(0)))
.or(named(RUN_METHOD_NAME).and(takesArguments(0)))
.or(named(GET_METHOD_NAME).and(takesArguments(0)));
}
@Override public String getMethodsInterceptor() {
return CALL_METHOD_INTERCEPTOR;
}
@Override public boolean isOverrideArgs() {
return false;
}
}
};
}
@Override protected ClassMatch enhanceClass() {
return byClassAnnotationMatch(new String[] {ANNOTATION_NAME});
}
}
ThreadLocal
java.lang.ThreadLocal无法复制上下文至子线程
java.lang.InheritableThreadLocal只有在线程创建时，把父线程的上下文复制到子线程
阿里开源的com.alibaba.ttl.TransmittableThreadLocal，可以在子线程执行任务前，把父线程的上下文复制到子线程
pom
<!-- https://mvnrepository.com/artifact/com.alibaba/transmittable-thread-local -->
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>transmittable-thread-local</artifactId>
<version>2.12.0</version>
</dependency>
跨进程传递上下文
TransmittableThreadLocal只能解决同进程内跨线程复制的问题，如果需要跨进程复制上下文，需要有拦截器。
以分布式链路id traceId的实现为例，
比如A服务http请求B服务，可能会通过Feign, RestTemplate, HttpClient等形式。需要为项目中用到的每个形式，定义拦截器，发送http请求时，从MDC取出traceId，放到http header里，key为trace-id。
B服务也需要有一个拦截器，从http header里拿出trace-id，放到MDC里。
如果是mq的场景，也是一样的
生产者需要从MDC取出traceId，放到mq的properties里面
消费者需要从mq的properties里，拿出traceId，放入MDC
