在Spring Boot和Logback中由不同的Classloader加载的类

Question

I ran into a wierd problem that I would like to understand. If someone has a solution that would be great, but I am searching actually for an untderstanding why this is happening:

I wrote a custom Logback Layout. I am extending ch.qos.logback.contrib.json.classic.JsonLayout and overriding addCustomDataToJsonMap . There I would like to add additional attributes if a certain type of argument is found in the logging events arguments list:

  protected void addCustomDataToJsonMap(Map<String, Object> map, ILoggingEvent event) {

    if (event.getArgumentArray() == null) {
      return;
    }

    for (Object argument : event.getArgumentArray()) {

      System.out.println(argument.getClass().getClassLoader()); // 1
      System.out.println(JsonAttribute.class.getClassLoader()); // 2

But the class of the object in the argument list (1) and the class of the static referenced one (2) are loaded by different classloaders as seen in the output:

org.springframework.boot.devtools.restart.classloader.RestartClassLoader@618157b2
sun.misc.Launcher$AppClassLoader@18b4aac2

Due to this I am not able to cast the object to the disired type and access its methods. The workaround I have in mind would be to access those via Reflecion, but I would rather use the real value.

I suppose that this is only a problem of my development environment, but as said in the first section I would really like to understand what is happening.

Edit:

As expected: When running the appliation in "production"-mode without the spring dev tools, the classes are loaded all by the same classloader.

Addendum:

The stacktrace of this function:

    at xxx.ExtendableJsonLayout.addCustomDataToJsonMap(ExtendableJsonLayout.java:26)
    at ch.qos.logback.contrib.json.classic.JsonLayout.toJsonMap(Unknown Source)
    at ch.qos.logback.contrib.json.classic.JsonLayout.toJsonMap(Unknown Source)
    at ch.qos.logback.contrib.json.JsonLayoutBase.doLayout(Unknown Source)
    at ch.qos.logback.core.encoder.LayoutWrappingEncoder.encode(LayoutWrappingEncoder.java:115)
    at ch.qos.logback.core.OutputStreamAppender.subAppend(OutputStreamAppender.java:230)
    at ch.qos.logback.core.OutputStreamAppender.append(OutputStreamAppender.java:102)
    at ch.qos.logback.core.UnsynchronizedAppenderBase.doAppend(UnsynchronizedAppenderBase.java:84)
    at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:51)
    at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:270)
    at ch.qos.logback.classic.Logger.callAppenders(Logger.java:257)
    at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:421)
    at ch.qos.logback.classic.Logger.filterAndLog_1(Logger.java:398)
    at ch.qos.logback.classic.Logger.error(Logger.java:526)
    at xxx.TestRunner.run(TestRunner.java:23)
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:813)
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:797)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:324)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248)
    at xxx.TestRunner.main(TestRunner.java:16)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)

My logback configuration for the appender:

  <appender name="json" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="ch.qos.logback.core.encoder.LayoutWrappingEncoder">
      <layout class="xxx.ExtendableJsonLayout">
        <jsonFormatter
          class="ch.qos.logback.contrib.jackson.JacksonJsonFormatter">
          <prettyPrint>true</prettyPrint>
        </jsonFormatter>
        <timestampFormat>yyyy-MM-dd' 'HH:mm:ss.SSS</timestampFormat>
      </layout>
    </encoder>
  </appender>

The JsonAttribute class:

@AllArgsConstructor
@Getter
public class JsonAttribute {

  private String key;
  private Object value;

  public static JsonAttribute attr(String key, Object value) {
    return new JsonAttribute(key, value);
  }

  @Override
  public String toString() {
    return key + "=" + value;
  }

Answer 1

I assume you are using Spring DevTools. They create a separate classloader for quicker application restart. This classloader is supposed to load your classes and is thrown away on restart instead of restarting the whole application. See the DevTools docs .

You should be able to configure it to exclude your custom Logback layout so that it is not reloaded on application restart (Sec. 20.2.6 in the linked docs).