Ensures unhealthy status in case a PoolingHttpClientConnectionManager is shut down, e.g. as result of OOM

HttpConnectionPoolHealthIndicator.java

import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.boot.actuate.health.Status;
import org.springframework.util.ReflectionUtils;

import java.lang.reflect.Field;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * Checks if none of the available {@link PoolingHttpClientConnectionManager} beans
 * are shut down, which can happen after an out-of-memory error, thus requiring
 * the application to be restarted.
 */
public class HttpConnectionPoolHealthIndicator implements HealthIndicator {

    private Map<String, AtomicBoolean> shutDownPerConnManager = new LinkedHashMap<>();

    public HttpConnectionPoolHealthIndicator(Map<String, PoolingHttpClientConnectionManager> connectionManagers) {
        if (connectionManagers.isEmpty()) return;
        Field isShutDownField = ReflectionUtils.findField(PoolingHttpClientConnectionManager.class, "isShutDown");
        ReflectionUtils.makeAccessible(isShutDownField);
        for (Map.Entry<String, PoolingHttpClientConnectionManager> entry : connectionManagers.entrySet()) {
            AtomicBoolean isShutdown = (AtomicBoolean) ReflectionUtils.getField(isShutDownField, entry.getValue());
            shutDownPerConnManager.put(entry.getKey(), isShutdown);
        }
    }

    @Override
    public Health health() {
        Health.Builder health = new Health.Builder(Status.UP);
        for (Map.Entry<String, AtomicBoolean> entry : shutDownPerConnManager.entrySet()) {
            String bean = entry.getKey();
            if (entry.getValue().get()) {
                health.down().withDetail(bean, "shutdown");
            } else {
                health.withDetail(bean, "running");
            }
        }
        return health.build();
    }
}