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

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：

加载Bean定义（从xml或者从@Import等）
处理BeanFactoryPostProcessor
实例化Bean
处理Bean的property注入
处理BeanPostProcessor

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里

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.getBeansOfType，beanFactory.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