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Collect information from Spring Boot actuator.

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package io.prometheus.client.spring.boot;

import io.prometheus.client.CollectorRegistry;
import io.prometheus.client.exporter.common.TextFormat;
import org.springframework.boot.actuate.endpoint.AbstractEndpoint;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.http.ResponseEntity;

import java.io.IOException;
import java.io.StringWriter;
import java.io.Writer;

import static org.springframework.http.HttpHeaders.CONTENT_TYPE;

@ConfigurationProperties("endpoints.prometheus")
class PrometheusEndpoint extends AbstractEndpoint> {

  private final CollectorRegistry collectorRegistry;

  PrometheusEndpoint(CollectorRegistry collectorRegistry) {
    super("prometheus");
    this.collectorRegistry = collectorRegistry;
  }

  @Override
  public ResponseEntity invoke() {
    try {
      Writer writer = new StringWriter();
      TextFormat.write004(writer, collectorRegistry.metricFamilySamples());
      return ResponseEntity.ok()
          .header(CONTENT_TYPE, TextFormat.CONTENT_TYPE_004)
          .body(writer.toString());
    } catch (IOException e) {
      // This actually never happens since StringWriter::write() doesn't throw any IOException
      throw new RuntimeException("Writing metrics failed", e);
    }
  }
}