1. 简介

Spring Boot的Endpoint主要是用来监控应用服务的运行状况，并集成在MVC中提供查看接口。内置的Endpoint比如HealthEndpoint会监控dist和db的状况

2. 分析

2.1 Endpoint源码

Endpoint是在包org.springframework.boot.actuate.endpoint下，其中invoke()是最主要的方法，用于返回监控的内容，isSensitive()用于权限控制。核心代码如下：

public interface Endpoint<T> {  
    String getId();  
    boolean isEnabled();  
    boolean isSensitive();  
    T invoke();  
}  

2.2 Endpoint加载实现

Endpoint的加载还是要依靠spring.factories实现的。Spring-boot-actutor包下的META-IN/spring.factories配置了EndpointAutoConfiguration。
spring.factories代码如下

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\  
org.springframework.boot.actuate.autoconfigure.AuditAutoConfiguration,\  
org.springframework.boot.actuate.autoconfigure.CacheStatisticsAutoConfiguration,\  
org.springframework.boot.actuate.autoconfigure.CrshAutoConfiguration,\  

2.3 EndpointAutoConfiguration自动加载类

EndpointAutoConfiguration（org.springframework.boot.actuate.autoconfigure包下）就会注入必要的Endpoint。有些Endpoint需要外部的收集类，比如TraceEndpoint。

 * Copyright 2012-2016 the original author or authors.

package org.springframework.boot.actuate.autoconfigure;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Map;

import liquibase.integration.spring.SpringLiquibase;
import org.flywaydb.core.Flyway;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.actuate.endpoint.AutoConfigurationReportEndpoint;
import org.springframework.boot.actuate.endpoint.BeansEndpoint;
import org.springframework.boot.actuate.endpoint.ConfigurationPropertiesReportEndpoint;
import org.springframework.boot.actuate.endpoint.DumpEndpoint;
import org.springframework.boot.actuate.endpoint.Endpoint;
import org.springframework.boot.actuate.endpoint.EndpointProperties;
import org.springframework.boot.actuate.endpoint.EnvironmentEndpoint;
import org.springframework.boot.actuate.endpoint.FlywayEndpoint;
import org.springframework.boot.actuate.endpoint.HealthEndpoint;
import org.springframework.boot.actuate.endpoint.InfoEndpoint;
import org.springframework.boot.actuate.endpoint.LiquibaseEndpoint;
import org.springframework.boot.actuate.endpoint.LoggersEndpoint;
import org.springframework.boot.actuate.endpoint.MetricsEndpoint;
import org.springframework.boot.actuate.endpoint.PublicMetrics;
import org.springframework.boot.actuate.endpoint.RequestMappingEndpoint;
import org.springframework.boot.actuate.endpoint.ShutdownEndpoint;
import org.springframework.boot.actuate.endpoint.TraceEndpoint;
import org.springframework.boot.actuate.health.HealthAggregator;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.boot.actuate.health.OrderedHealthAggregator;
import org.springframework.boot.actuate.info.InfoContributor;
import org.springframework.boot.actuate.trace.InMemoryTraceRepository;
import org.springframework.boot.actuate.trace.TraceRepository;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionEvaluationReport;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.SearchStrategy;
import org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration;
import org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.boot.logging.LoggingSystem;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.web.servlet.handler.AbstractHandlerMethodMapping;

/**
 * {@link EnableAutoConfiguration Auto-configuration} for common management
 * {@link Endpoint}s.
 *
 * @author Dave Syer
 * @author Phillip Webb
 * @author Greg Turnquist
 * @author Christian Dupuis
 * @author Stephane Nicoll
 * @author Eddú Meléndez
 * @author Meang Akira Tanaka
 * @author Ben Hale
 */
@Configuration
@AutoConfigureAfter({ FlywayAutoConfiguration.class, LiquibaseAutoConfiguration.class })
@EnableConfigurationProperties(EndpointProperties.class)
public class EndpointAutoConfiguration {

    private final HealthAggregator healthAggregator;

    private final Map<String, HealthIndicator> healthIndicators;

    private final List<InfoContributor> infoContributors;

    private final Collection<PublicMetrics> publicMetrics;

    private final TraceRepository traceRepository;

    public EndpointAutoConfiguration(ObjectProvider<HealthAggregator> healthAggregator,
            ObjectProvider<Map<String, HealthIndicator>> healthIndicators,
            ObjectProvider<List<InfoContributor>> infoContributors,
            ObjectProvider<Collection<PublicMetrics>> publicMetrics,
            ObjectProvider<TraceRepository> traceRepository) {
        this.healthAggregator = healthAggregator.getIfAvailable();
        this.healthIndicators = healthIndicators.getIfAvailable();
        this.infoContributors = infoContributors.getIfAvailable();
        this.publicMetrics = publicMetrics.getIfAvailable();
        this.traceRepository = traceRepository.getIfAvailable();
    }

    @Bean
    @ConditionalOnMissingBean
    public EnvironmentEndpoint environmentEndpoint() {
        return new EnvironmentEndpoint();
    }

    @Bean
    @ConditionalOnMissingBean
    public HealthEndpoint healthEndpoint() {
        return new HealthEndpoint(
                this.healthAggregator == null ? new OrderedHealthAggregator()
                        : this.healthAggregator,
                this.healthIndicators == null
                        ? Collections.<String, HealthIndicator>emptyMap()
                        : this.healthIndicators);
    }

    @Bean
    @ConditionalOnMissingBean
    public BeansEndpoint beansEndpoint() {
        return new BeansEndpoint();
    }

    @Bean
    @ConditionalOnMissingBean
    public InfoEndpoint infoEndpoint() throws Exception {
        return new InfoEndpoint(this.infoContributors == null
                ? Collections.<InfoContributor>emptyList() : this.infoContributors);
    }

    @Bean
    @ConditionalOnBean(LoggingSystem.class)
    @ConditionalOnMissingBean
    public LoggersEndpoint loggersEndpoint(LoggingSystem loggingSystem) {
        return new LoggersEndpoint(loggingSystem);
    }

    @Bean
    @ConditionalOnMissingBean
    public MetricsEndpoint metricsEndpoint() {
        List<PublicMetrics> publicMetrics = new ArrayList<PublicMetrics>();
        if (this.publicMetrics != null) {
            publicMetrics.addAll(this.publicMetrics);
        }
        Collections.sort(publicMetrics, AnnotationAwareOrderComparator.INSTANCE);
        return new MetricsEndpoint(publicMetrics);
    }

    @Bean
    @ConditionalOnMissingBean
    public TraceEndpoint traceEndpoint() {
        return new TraceEndpoint(this.traceRepository == null
                ? new InMemoryTraceRepository() : this.traceRepository);
    }

    @Bean
    @ConditionalOnMissingBean
    public DumpEndpoint dumpEndpoint() {
        return new DumpEndpoint();
    }

    @Bean
    @ConditionalOnBean(ConditionEvaluationReport.class)
    @ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
    public AutoConfigurationReportEndpoint autoConfigurationReportEndpoint() {
        return new AutoConfigurationReportEndpoint();
    }

    @Bean
    @ConditionalOnMissingBean
    public ShutdownEndpoint shutdownEndpoint() {
        return new ShutdownEndpoint();
    }

    @Bean
    @ConditionalOnMissingBean
    public ConfigurationPropertiesReportEndpoint configurationPropertiesReportEndpoint() {
        return new ConfigurationPropertiesReportEndpoint();
    }

    @Configuration
    @ConditionalOnBean(Flyway.class)
    @ConditionalOnClass(Flyway.class)
    static class FlywayEndpointConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public FlywayEndpoint flywayEndpoint(Map<String, Flyway> flyways) {
            return new FlywayEndpoint(flyways);
        }

    }

    @Configuration
    @ConditionalOnBean(SpringLiquibase.class)
    @ConditionalOnClass(SpringLiquibase.class)
    static class LiquibaseEndpointConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public LiquibaseEndpoint liquibaseEndpoint(
                Map<String, SpringLiquibase> liquibases) {
            return new LiquibaseEndpoint(liquibases);
        }

    }

    @Configuration
    @ConditionalOnClass(AbstractHandlerMethodMapping.class)
    protected static class RequestMappingEndpointConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public RequestMappingEndpoint requestMappingEndpoint() {
            RequestMappingEndpoint endpoint = new RequestMappingEndpoint();
            return endpoint;
        }

    }

}

TraceEndpoint会记录每次请求的Request和Response的状态，需要嵌入到Request的流程中，这里就主要用到了3个类。

2.4 TraceRepository用于保存和获取Request和Response的状态。

 * Copyright 2012-2015 the original author or authors.

package org.springframework.boot.actuate.trace;

import java.util.List;
import java.util.Map;

/**
 * A repository for {@link Trace}s.
 *
 * @author Dave Syer
 */
public interface TraceRepository {

    /**
     * Find all {@link Trace} objects contained in the repository.
     * @return the results
     */
    List<Trace> findAll();

    /**
     * Add a new {@link Trace} object at the current time.
     * @param traceInfo trace information
     */
    void add(Map<String, Object> traceInfo);

}

2.5 WebRequestTraceFilter用于嵌入web request，收集请求的状态并保存在TraceRepository中。

2.6 TraceEndpoint，invoke()方法直接调用TraceRepository保存的数据。

 * Copyright 2012-2016 the original author or authors.

package org.springframework.boot.actuate.endpoint;

import java.util.List;

import org.springframework.boot.actuate.trace.Trace;
import org.springframework.boot.actuate.trace.TraceRepository;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.util.Assert;

/**
 * {@link Endpoint} to expose {@link Trace} information.
 *
 * @author Dave Syer
 */
@ConfigurationProperties(prefix = "endpoints.trace")
public class TraceEndpoint extends AbstractEndpoint<List<Trace>> {

    private final TraceRepository repository;

    /**
     * Create a new {@link TraceEndpoint} instance.
     * @param repository the trace repository
     */
    public TraceEndpoint(TraceRepository repository) {
        super("trace");
        Assert.notNull(repository, "Repository must not be null");
        this.repository = repository;
    }

    @Override
    public List<Trace> invoke() {
        return this.repository.findAll();
    }

}

3. 总结

很多spring-starter都是基于是spring.factories的配置实现，可以完美的提高复用性，模块化开发。可以利用好endpoint实现不少的监控统计功能