MDC ( Mapped Diagnostic Contexts ) 有了日志之后，我们就可以追踪各种线上问题。但是，在分布式系统中，各种无关日志穿行其中，导致我们可能无法直接定位整个操作流程。因此，我们可能需要对一个用户的操作流程进行归类标记，比如使用线程+时间戳，或者用户身份标识等；如此，我们可以从大量日志信息中grep出某个用户的操作流程，或者某个时间的流转记录。其目的是为了便于我们诊断线上问题而出现的方法工具类。虽然，Slf4j 是用来适配其他的日志具体实现包的，但是针对 MDC功能，目前只有logback 以及 log4j 支持。
MDC
package org.slf4j;
import java.io.Closeable;
import java.util.Map;
import org.slf4j.helpers.NOPMDCAdapter;
import org.slf4j.helpers.BasicMDCAdapter;
import org.slf4j.helpers.Util;
import org.slf4j.impl.StaticMDCBinder;
import org.slf4j.spi.MDCAdapter;
public class MDC {
static final String NULL_MDCA_URL = "http://www.slf4j.org/codes.html#null_MDCA";
static final String NO_STATIC_MDC_BINDER_URL = "http://www.slf4j.org/codes.html#no_static_mdc_binder";
static MDCAdapter mdcAdapter;
public static class MDCCloseable implements Closeable {
private final String key;
private MDCCloseable(String key) {
this.key = key;
}
public void close() {
MDC.remove(this.key);
}
}
private MDC() {
}
static {
try {
mdcAdapter = StaticMDCBinder.SINGLETON.getMDCA();
} catch (NoClassDefFoundError ncde) {
mdcAdapter = new NOPMDCAdapter();
String msg = ncde.getMessage();
if (msg != null && msg.indexOf("StaticMDCBinder") != -1) {
Util.report("Failed to load class \"org.slf4j.impl.StaticMDCBinder\".");
Util.report("Defaulting to no-operation MDCAdapter implementation.");
Util.report("See " + NO_STATIC_MDC_BINDER_URL + " for further details.");
} else {
throw ncde;
}
} catch (Exception e) {
// we should never get here
Util.report("MDC binding unsuccessful.", e);
}
}
public static void put(String key, String val) throws IllegalArgumentException {
if (key == null) {
throw new IllegalArgumentException("key parameter cannot be null");
}
if (mdcAdapter == null) {
throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
}
mdcAdapter.put(key, val);
}
public static MDCCloseable putCloseable(String key, String val) throws IllegalArgumentException {
put(key, val);
return new MDCCloseable(key);
}
public static String get(String key) throws IllegalArgumentException {
if (key == null) {
throw new IllegalArgumentException("key parameter cannot be null");
}
if (mdcAdapter == null) {
throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
}
return mdcAdapter.get(key);
}
public static void remove(String key) throws IllegalArgumentException {
if (key == null) {
throw new IllegalArgumentException("key parameter cannot be null");
}
if (mdcAdapter == null) {
throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
}
mdcAdapter.remove(key);
}
public static void clear() {
if (mdcAdapter == null) {
throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
}
mdcAdapter.clear();
}
public static Map<String, String> getCopyOfContextMap() {
if (mdcAdapter == null) {
throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
}
return mdcAdapter.getCopyOfContextMap();
}
public static void setContextMap(Map<String, String> contextMap) {
if (mdcAdapter == null) {
throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
}
mdcAdapter.setContextMap(contextMap);
}
public static MDCAdapter getMDCAdapter() {
return mdcAdapter;
}
}
简单的demo
package com.alibaba.otter.canal.common;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;
public class LogTest {
private static final Logger logger = LoggerFactory.getLogger(LogTest.class);
public static void main(String[] args) {
MDC.put("THREAD_ID", String.valueOf(Thread.currentThread().getId()));
logger.info("纯字符串信息的info级别日志");
}
}
logback.xml 配置
<configuration scan="true" scanPeriod=" 5 seconds">
<jmxConfigurator />
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{56} %X{THREAD_ID} - %msg%n
</pattern>
</encoder>
</appender>
<root level="INFO">
<appender-ref ref="STDOUT"/>
</root>
</configuration
对应的输出日志 可以看到输出了THREAD_ID
2016-12-08 14:59:32.855 [main] INFO com.alibaba.otter.canal.common.LogTest THREAD_ID 1 - 纯字符串信息的info级别日志
slf4j只是起到适配的作用 故查看实现类LogbackMDCAdapter
属性
final InheritableThreadLocal<Map<String, String>> copyOnInheritThreadLocal = new InheritableThreadLocal<Map<String, String>>();
InheritableThreadLocal 该类扩展了 ThreadLocal，为子线程提供从父线程那里继承的值：在创建子线程时，子线程会接收所有可继承的线程局部变量的初始值，以获得父线程所具有的值。通常，子线程的值与父线程的值是一致的；但是，通过重写这个类中的 childValue 方法，子线程的值可以作为父线程值的一个任意函数。
当必须将变量（如用户 ID 和 事务 ID）中维护的每线程属性（per-thread-attribute）自动传送给创建的所有子线程时，应尽可能地采用可继承的线程局部变量，而不是采用普通的线程局部变量
验证一下
package com.alibaba.otter.canal.parse.driver.mysql;
import org.junit.Test;
public class TestInheritableThreadLocal {
@Test
public void testThreadLocal() {
final ThreadLocal<String> local = new ThreadLocal<String>();
work(local);
}
@Test
public void testInheritableThreadLocal() {
final ThreadLocal<String> local = new InheritableThreadLocal<String>();
work(local);
}
private void work(final ThreadLocal<String> local) {
local.set("a");
System.out.println(Thread.currentThread() + "," + local.get());
Thread t = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(Thread.currentThread() + "," + local.get());
local.set("b");
System.out.println(Thread.currentThread() + "," + local.get());
}
});
t.start();
try {
t.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread() + "," + local.get());
}
}
分别运行得到的输出结果
ThreadLocal 存贮输出结果
Thread[main,5,main],a
Thread[Thread-0,5,main],null
Thread[Thread-0,5,main],b
Thread[main,5,main],a
InheritableThreadLocal存贮输出结果
Thread[main,5,main],a
Thread[Thread-0,5,main],a
Thread[Thread-0,5,main],b
Thread[main,5,main],a
输出结果说明一切 对于参数传递十分有用 我知道 canal的源码中用到了MDC
在 CanalServerWithEmbedded 中的 start 和stop等方法中都有用到
public void start(final String destination) {
final CanalInstance canalInstance = canalInstances.get(destination);
if (!canalInstance.isStart()) {
try {
MDC.put("destination", destination);
canalInstance.start();
logger.info("start CanalInstances[{}] successfully", destination);
} finally {
MDC.remove("destination");
}
}
}
public void stop(String destination) {
CanalInstance canalInstance = canalInstances.remove(destination);
if (canalInstance != null) {
if (canalInstance.isStart()) {
try {
MDC.put("destination", destination);
canalInstance.stop();
logger.info("stop CanalInstances[{}] successfully", destination);
} finally {
MDC.remove("destination");
}
}
}
}
