BackendSelectionPolicyFactory.java

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// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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package org.apache.doris.resource;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import java.util.Iterator;
import java.util.ServiceLoader;

/**
 * Discovers a {@link BackendSelectionPolicy} via {@link ServiceLoader}. If no custom
 * provider is on the classpath, falls back to the interface's no-op defaults.
 */
public class BackendSelectionPolicyFactory {
    private static final Logger LOG = LogManager.getLogger(BackendSelectionPolicyFactory.class);

    private static final BackendSelectionPolicy DEFAULT = new BackendSelectionPolicy() {
    };

    private static volatile BackendSelectionPolicy instance;

    public static BackendSelectionPolicy get() {
        if (instance == null) {
            synchronized (BackendSelectionPolicyFactory.class) {
                if (instance == null) {
                    instance = load(BackendSelectionPolicyFactory.class.getClassLoader());
                }
            }
        }
        return instance;
    }

    static BackendSelectionPolicy load(ClassLoader classLoader) {
        ServiceLoader<BackendSelectionPolicy> loader =
                ServiceLoader.load(BackendSelectionPolicy.class, classLoader);
        Iterator<BackendSelectionPolicy> it = loader.iterator();
        if (it.hasNext()) {
            BackendSelectionPolicy policy = it.next();
            LOG.info("Loaded BackendSelectionPolicy implementation: {}", policy.getClass().getName());
            return policy;
        }
        LOG.info("No BackendSelectionPolicy implementation found, using no-op backend selection policy");
        return DEFAULT;
    }
}