深入Spring Boot:那些注入不了的Spring占位符(${}表达式)

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Spring里的占位符

spring里的占位符通常表现的形式是:

<bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp.BasicDataSource">
    <property name="url" value="${jdbc.url}"/>
</bean>

或者

@Configuration
@ImportResource("classpath:/com/acme/properties-config.xml")
public class AppConfig {
    @Value("${jdbc.url}")
    private String url;
}

Spring应用在有时会出现占位符配置没有注入,原因可能是多样的。

本文介绍两种比较复杂的情况。

占位符是在Spring生命周期的什么时候处理的

Spirng在生命周期里关于Bean的处理大概可以分为下面几步:

  1. 加载Bean定义(从xml或者从@Import等)
  2. 处理BeanFactoryPostProcessor
  3. 实例化Bean
  4. 处理Bean的property注入
  5. 处理BeanPostProcessor

技术分享

当然这只是比较理想的状态,实际上因为Spring Context在构造时,也需要创建很多内部的Bean,应用在接口实现里也会做自己的各种逻辑,整个流程会非常复杂。

那么占位符(${}表达式)是在什么时候被处理的?

  • 实际上是在org.springframework.context.support.PropertySourcesPlaceholderConfigurer里处理的,它会访问了每一个bean的BeanDefinition,然后做占位符的处理
  • PropertySourcesPlaceholderConfigurer实现了BeanFactoryPostProcessor接口
  • PropertySourcesPlaceholderConfigurer的 order是Ordered.LOWEST_PRECEDENCE,也就是最低优先级的

结合上面的Spring的生命周期,如果Bean的创建和使用在PropertySourcesPlaceholderConfigurer之前,那么就有可能出现占位符没有被处理的情况。

例子1:Mybatis 的 MapperScannerConfigurer引起的占位符没有处理

例子代码:mybatis-demo.zip

  • 首先应用自己在代码里创建了一个DataSource,其中${db.user}是希望从application.properties里注入的。代码在运行时会打印出user的实际值。

    @Configuration
    public class MyDataSourceConfig {
        @Bean(name = "dataSource1")
        public DataSource dataSource1(@Value("${db.user}") String user) {
            System.err.println("user: " + user);
            JdbcDataSource ds = new JdbcDataSource();
            ds.setURL("jdbc:h2:˜/test");
            ds.setUser(user);
            return ds;
        }
    }

     

  • 然后应用用代码的方式来初始化mybatis相关的配置,依赖上面创建的DataSource对象

    @Configuration
    public class MybatisConfig1 {
    
        @Bean(name = "sqlSessionFactory1")
        public SqlSessionFactory sqlSessionFactory1(DataSource dataSource1) throws Exception {
            SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
            org.apache.ibatis.session.Configuration ibatisConfiguration = new org.apache.ibatis.session.Configuration();
            sqlSessionFactoryBean.setConfiguration(ibatisConfiguration);
    
            sqlSessionFactoryBean.setDataSource(dataSource1);
            sqlSessionFactoryBean.setTypeAliasesPackage("sample.mybatis.domain");
            return sqlSessionFactoryBean.getObject();
        }
    
        @Bean
        MapperScannerConfigurer mapperScannerConfigurer(SqlSessionFactory sqlSessionFactory1) {
            MapperScannerConfigurer mapperScannerConfigurer = new MapperScannerConfigurer();
            mapperScannerConfigurer.setSqlSessionFactoryBeanName("sqlSessionFactory1");
            mapperScannerConfigurer.setBasePackage("sample.mybatis.mapper");
            return mapperScannerConfigurer;
        }
    }

     

当代码运行时,输出结果是:

user: ${db.user}

 

为什么会user这个变量没有被注入?

分析下Bean定义,可以发现MapperScannerConfigurer它实现了BeanDefinitionRegistryPostProcessor。这个接口在是Spring扫描Bean定义时会回调的,远早于BeanFactoryPostProcessor

所以原因是:

  • MapperScannerConfigurer它实现了BeanDefinitionRegistryPostProcessor,所以它会Spring的早期会被创建
  • 从bean的依赖关系来看,mapperScannerConfigurer依赖了sqlSessionFactory1,sqlSessionFactory1依赖了dataSource1
  • MyDataSourceConfig里的dataSource1被提前初始化,没有经过PropertySourcesPlaceholderConfigurer的处理,所以@Value("${db.user}") String user 里的占位符没有被处理

要解决这个问题,可以在代码里,显式来处理占位符:

environment.resolvePlaceholders("${db.user}")

package org.mybatis.spring.mapper;

import static org.springframework.util.Assert.notNull;

import java.lang.annotation.Annotation;
import java.util.Map;

import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionTemplate;
import org.springframework.beans.PropertyValue;
import org.springframework.beans.PropertyValues;
import org.springframework.beans.factory.BeanNameAware;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.PropertyResourceConfigurer;
import org.springframework.beans.factory.config.TypedStringValue;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
import org.springframework.beans.factory.support.BeanNameGenerator;
import org.springframework.beans.factory.support.DefaultListableBeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.util.StringUtils;

/**
 * BeanDefinitionRegistryPostProcessor that searches recursively starting from a base package for
 * interfaces and registers them as {@code MapperFactoryBean}. Note that only interfaces with at
 * least one method will be registered; concrete classes will be ignored.
 * <p>
 * This class was a {code BeanFactoryPostProcessor} until 1.0.1 version. It changed to  
 * {@code BeanDefinitionRegistryPostProcessor} in 1.0.2. See https://jira.springsource.org/browse/SPR-8269
 * for the details.
 * <p>
 * The {@code basePackage} property can contain more than one package name, separated by either
 * commas or semicolons.
 * <p>
 * This class supports filtering the mappers created by either specifying a marker interface or an
 * annotation. The {@code annotationClass} property specifies an annotation to search for. The
 * {@code markerInterface} property specifies a parent interface to search for. If both properties
 * are specified, mappers are added for interfaces that match <em>either</em> criteria. By default,
 * these two properties are null, so all interfaces in the given {@code basePackage} are added as
 * mappers.
 * <p>
 * This configurer enables autowire for all the beans that it creates so that they are
 * automatically autowired with the proper {@code SqlSessionFactory} or {@code SqlSessionTemplate}.
 * If there is more than one {@code SqlSessionFactory} in the application, however, autowiring
 * cannot be used. In this case you must explicitly specify either an {@code SqlSessionFactory} or
 * an {@code SqlSessionTemplate} to use via the <em>bean name</em> properties. Bean names are used
 * rather than actual objects because Spring does not initialize property placeholders until after
 * this class is processed. 
 * <p>
 * Passing in an actual object which may require placeholders (i.e. DB user password) will fail. 
 * Using bean names defers actual object creation until later in the startup
 * process, after all placeholder substituation is completed. However, note that this configurer
 * does support property placeholders of its <em>own</em> properties. The <code>basePackage</code>
 * and bean name properties all support <code>${property}</code> style substitution.
 * <p>
 * Configuration sample:
 * <p>
 *
 * <pre class="code">
 * {@code
 *   <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
 *       <property name="basePackage" value="org.mybatis.spring.sample.mapper" />
 *       <!-- optional unless there are multiple session factories defined -->
 *       <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
 *   </bean>
 * }
 * </pre>
 *
 * @author Hunter Presnall
 * @author Eduardo Macarron
 *
 * @see MapperFactoryBean
 * @see ClassPathMapperScanner
 */
public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {

  private String basePackage;

  private boolean addToConfig = true;

  private SqlSessionFactory sqlSessionFactory;

  private SqlSessionTemplate sqlSessionTemplate;

  private String sqlSessionFactoryBeanName;

  private String sqlSessionTemplateBeanName;

  private Class<? extends Annotation> annotationClass;

  private Class<?> markerInterface;

  private ApplicationContext applicationContext;

  private String beanName;

  private boolean processPropertyPlaceHolders;

  private BeanNameGenerator nameGenerator;

 

例子2:Spring boot自身实现问题,导致Bean被提前初始化

例子代码:demo.zip

Spring Boot里提供了@ConditionalOnBean,这个方便用户在不同条件下来创建bean。里面提供了判断是否存在bean上有某个注解的功能。

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {
    /**
     * The annotation type decorating a bean that should be checked. The condition matches
     * when any of the annotations specified is defined on a bean in the
     * {@link ApplicationContext}.
     * @return the class-level annotation types to check
     */
    Class<? extends Annotation>[] annotation() default {};

 


比如用户自己定义了一个Annotation:

@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
}

 

然后用下面的写法来创建abc这个bean,意思是当用户显式使用了@MyAnnotation(比如放在main class上),才会创建这个bean。

@Configuration
public class MyAutoConfiguration {
    @Bean
    // if comment this line, it will be fine.
    @ConditionalOnBean(annotation = { MyAnnotation.class })
    public String abc() {
        return "abc";
    }
}

这个功能很好,但是在spring boot 1.4.5 版本之前都有问题,会导致FactoryBean提前初始化。

在例子里,通过xml创建了javaVersion这个bean,想获取到Java的版本号。这里使用的是spring提供的一个调用static函数创建bean的技巧。

<bean id="sysProps" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
  <property name="targetClass" value="java.lang.System" />
  <property name="targetMethod" value="getProperties" />
</bean>

<bean id="javaVersion" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
  <property name="targetObject" ref="sysProps" />
  <property name="targetMethod" value="getProperty" />
  <property name="arguments" value="${java.version.key}" />
</bean>

我们在代码里获取到这个javaVersion,然后打印出来:

@SpringBootApplication
@ImportResource("classpath:/demo.xml")
public class DemoApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(DemoApplication.class, args);
        System.err.println(context.getBean("javaVersion"));
    }
}

在实际运行时,发现javaVersion的值是null。

这个其实是spring boot的锅,要搞清楚这个问题,先要看@ConditionalOnBean的实现。

  • @ConditionalOnBean实际上是在ConfigurationClassPostProcessor里被处理的,它实现了BeanDefinitionRegistryPostProcessor
  • BeanDefinitionRegistryPostProcessor是在spring早期被处理的
  • @ConditionalOnBean的具体处理代码在org.springframework.boot.autoconfigure.condition.OnBeanCondition
  • OnBeanCondition在获取bean的Annotation时,调用了beanFactory.getBeanNamesForAnnotation

    private String[] getBeanNamesForAnnotation(
        ConfigurableListableBeanFactory beanFactory, String type,
        ClassLoader classLoader, boolean considerHierarchy) throws LinkageError {
      String[] result = NO_BEANS;
      try {
        @SuppressWarnings("unchecked")
        Class<? extends Annotation> typeClass = (Class<? extends Annotation>) ClassUtils
            .forName(type, classLoader);
        result = beanFactory.getBeanNamesForAnnotation(typeClass);

     

  • beanFactory.getBeanNamesForAnnotation 会导致FactoryBean提前初始化,创建出javaVersion里,传入的${java.version.key}没有被处理,值为null。

  • spring boot 1.4.5 修复了这个问题:https://github.com/spring-projects/spring-boot/issues/8269

实现spring boot starter要注意不能导致bean提前初始化

用户在实现spring boot starter时,通常会实现Spring的一些接口,比如BeanFactoryPostProcessor接口,在处理时,要注意不能调用类似beanFactory.getBeansOfTypebeanFactory.getBeanNamesForAnnotation 这些函数,因为会导致一些bean提前初始化。

而上面有提到PropertySourcesPlaceholderConfigurer的order是最低优先级的,所以用户自己实现的BeanFactoryPostProcessor接口在被回调时很有可能占位符还没有被处理。

对于用户自己定义的@ConfigurationProperties对象的注入,可以用类似下面的代码:

@ConfigurationProperties(prefix = "spring.my")
public class MyProperties {
    String key;
}

  

 
public static MyProperties buildMyProperties(ConfigurableEnvironment environment) {
  MyProperties myProperties = new MyProperties();

  if (environment != null) {
    MutablePropertySources propertySources = environment.getPropertySources();
    new RelaxedDataBinder(myProperties, "spring.my").bind(new PropertySourcesPropertyValues(propertySources));
  }

  return myProperties;
}

 

总结

  • 占位符(${}表达式)是在PropertySourcesPlaceholderConfigurer里处理的,也就是BeanFactoryPostProcessor接口
  • spring的生命周期是比较复杂的事情,在实现了一些早期的接口时要小心,不能导致spring bean提前初始化
  • 在早期的接口实现里,如果想要处理占位符,可以利用spring自身的api,比如 environment.resolvePlaceholders("${db.user}")
 
 http://blog.csdn.net/hengyunabc/article/details/75453307

 

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