Does Skywalk support developing custom Watchers to dynamically read agent configurations in plugins

[llluckyou]

I have developed my own Skywalking plugin and I want to read agent configuration in interceptor. What should I do

I tried writing two Java classes, OcpTracerDowngradeWatcher and OcpTracerDowngradeService, During the process, reference was made to the apm-trace-ignore-plugin-9.4.0.jar, then, I obtain agent configuration through ServiceManager.INSTANCE.findService(OcpTracerDowngradeService.class), I found that this doesn't work. I encountered some issues with the class loader, I want to know if the official supports this during the design process ?

/*
 * Ant Group
 * Copyright (c) 2004-2025 All Rights Reserved.
 */
package org.apache.skywalking.apm.plugin.ocp.watcher;

import org.apache.skywalking.apm.agent.core.conf.dynamic.AgentConfigChangeWatcher;
import org.apache.skywalking.apm.agent.core.logging.api.ILog;
import org.apache.skywalking.apm.agent.core.logging.api.LogManager;

import java.util.concurrent.atomic.AtomicReference;

/**
 * @author renfang
 * @version OcpTracerDowngradeWatcher.java, v 0.1 2025年07月08日 上午2:27 renfang
 */
public class OcpTracerDowngradeWatcher extends AgentConfigChangeWatcher {

    private static final ILog LOGGER = LogManager.getLogger(OcpTracerDowngradeWatcher.class);

    private final AtomicReference<String> ocpTracerDowngrade;

    public OcpTracerDowngradeWatcher(final String propertyKey) {
        super(propertyKey);
        this.ocpTracerDowngrade = new AtomicReference(getDefaultValue());
    }

    private void activeSetting(String config) {
        if (LOGGER.isDebugEnable()) {
            LOGGER.debug("Updating ocpTracerDowngrade config: {}", config);
        }

        this.ocpTracerDowngrade.set(config);
    }

    @Override
    public void notify(final ConfigChangeEvent value) {
        if (EventType.DELETE.equals(value.getEventType())) {
            activeSetting(getDefaultValue());
        } else {
            activeSetting(value.getNewValue());
        }
    }

    @Override
    public String value() {
        return ocpTracerDowngrade.get();
    }

    private String getDefaultValue() {
        return Boolean.FALSE.toString();
    }

    public String getOcpTracerDowngrade() {
        return ocpTracerDowngrade.get();
    }

}

/*
 * Ant Group
 * Copyright (c) 2004-2025 All Rights Reserved.
 */
package org.apache.skywalking.apm.plugin.ocp.watcher;

import org.apache.skywalking.apm.agent.core.boot.BootService;
import org.apache.skywalking.apm.agent.core.boot.DefaultImplementor;
import org.apache.skywalking.apm.agent.core.boot.ServiceManager;
import org.apache.skywalking.apm.agent.core.conf.dynamic.ConfigurationDiscoveryService;
import org.apache.skywalking.apm.util.StringUtil;

/**
 * OcpTracer降级服务
 *
 * @author renfang
 * @version OcpTracerDowngradeService.java, v 0.1 2025年07月09日 下午2:21 renfang
 */
@DefaultImplementor
public class OcpTracerDowngradeService implements BootService {

    private OcpTracerDowngradeWatcher ocpTracerDowngradeWatcher;

    private volatile boolean downgradeSwitch = false;

    @Override
    public void prepare() {
    }

    @Override
    public void boot() {
        ocpTracerDowngradeWatcher = new OcpTracerDowngradeWatcher("agent.ocp.tracer_downgrade");
        ServiceManager.INSTANCE.findService(ConfigurationDiscoveryService.class).registerAgentConfigChangeWatcher(ocpTracerDowngradeWatcher);
        handleTracerDowngradeChanged();
    }

    @Override
    public void onComplete() {
    }

    @Override
    public void shutdown() {
    }

    /**
     * 获取降级开关
     *
     * @return 降级开关
     */
    public boolean getDowngradeSwitch() {
        return downgradeSwitch;
    }

    /**
     * 更新降级开关
     */
    void handleTracerDowngradeChanged() {
        if (StringUtil.isNotBlank(ocpTracerDowngradeWatcher.getOcpTracerDowngrade())) {
            downgradeSwitch = Boolean.valueOf(ocpTracerDowngradeWatcher.getOcpTracerDowngrade());
        } else {
            downgradeSwitch = false;
        }
    }
}

[wu-sheng]

If yoi wrote plugins, you need to get clear head of classloader by yourself.
This is not our design issue.

[llluckyou]

Thanks for your reply.

What I mean is not that there are design issues, During the debugging process, I found that there are two AgentClassLoader, The class object in Map<Class, BootService> ServiceManager.INSTANCE.bootedServices is loaded by one of AgentClassLoader, but the OcpTracerDowngradeService I wrote load by another one, So when I obtained the Service through

ServiceManager.INSTANCE.findService((Class<OcpTracerDowngradeService>) AgentClassLoader.getDefault().loadClass("org.apache.skywalking.apm.plugin.ocp.watcher.OcpTracerDowngradeService"))

the following exception occurred

java.lang.ClassCastException: org.apache.skywalking.apm.plugin.ocp.watcher.OcpTracerDowngradeService cannot be cast to org.apache.skywalking.apm.plugin.ocp.watcher.OcpTracerDowngradeService

I have checked a lot of information, including the official website, and there is almost no mention of whether Skywalking can customize dynamic loading agent configuration. Therefore, I would like to know if the official supports this approach during the design process.

[wu-sheng]

That is common and typical issue. You should not create and use service in the plugin, which has to be injected per its classloader.
If you need to coordinate crosss classloader, either enject the shared class into parent classloader and share it, or use basic and JVM included class type.

[llluckyou]

I found an article on the official website that mentioned this problem. It seems that calling BootService in Interceptor is not supported

https://skywalking.apache.org/zh/2023-09-17-integrating-skywalking-with-arthas/