Spring编程常见错误50例-Spring Bean依赖注入常见错误（下）

@Value没有注入预期的值

问题

对于@Value可以装配多种类型的数据：

• 装配对象：

@Value("#{student}")
private Student student;@Bean
public Student student(){Student student = createStudent(1, "xie");return student;
}

• 装配字符串：

@Value("我是字符串")
private String text;

• 注入系统参数、环境变量或者配置文件中的值：

@Value("${ip}")
private String ip

• 注入其他Bean属性：

@Value("#{student.name}")  // student是bean的ID
private String name;

但是使用该注解时遇到以下场景会出现问题：在控制器类中引用配置类中的属性时部分值返回错误

username=admin
password=pass

@RestController
@Slf4j
public class ValueTestController {@Value("${username}")private String username;@Value("${password}")private String password;@RequestMapping(path = "user", method = RequestMethod.GET)public String getUser(){return username + ","  + ", " + password;  // username返回的是运行这段程序的计算机用户名，password能正确返回};}

原因

从下面代码中可以看到@Value的工作分为三个核心步骤：

// DefaultListableBeanFactory#doResolveDependency
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {// ...// ①寻找@ValueObject value = getAutowireCandidateResolver().getSuggestedValue(descriptor);if (value != null) {// ②解析value值if (value instanceof String) {String strVal = resolveEmbeddedValue((String) value);BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);value = evaluateBeanDefinitionString(strVal, bd);}// ③转化Value解析的结果到装配的类型TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());try {return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());}catch (UnsupportedOperationException ex) {...}}// ...
}

• 寻找@Value：判断属性字段是否标记为@Value

// QualifierAnnotationAutowireCandidateResolver#findValue
@Nullable
protected Object findValue(Annotation[] annotationsToSearch) {if (annotationsToSearch.length > 0) {   // qualifier annotations have to be localAnnotationAttributes attr = AnnotatedElementUtils.getMergedAnnotationAttributes(// valueAnnotationType即为@ValueAnnotatedElementUtils.forAnnotations(annotationsToSearch), this.valueAnnotationType);if (attr != null) {return extractValue(attr);}}return null;
}

• 解析@Value的字符串值：如果字段标记了@Value，则可拿到对应的字符串值，然后就可以根据字符串值去做解析，解析结果可能是字符串，也可能是对象

• 将解析结果转化为要装配的对象的类型：

分析完对应的步骤后，可以定位到问题原因在解析@Value指定字符串过程中，对${xxx}的查找不局限在application.properties，而是针对多个源，这些源在启动时被有序固定，所以在查找时也是按序查找的。当查找到systemEnvironment时发有个username和配置文件中的重合：

// PropertySourcesPropertyResolver#getProperty
@Nullable
protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {if (this.propertySources != null) {for (PropertySource<?> propertySource : this.propertySources) {if (logger.isTraceEnabled()) {logger.trace("Searching for key '" + key + "' in PropertySource '" +propertySource.getName() + "'");}Object value = propertySource.getProperty(key);if (value != null) {if (resolveNestedPlaceholders && value instanceof String) {value = resolveNestedPlaceholders((String) value);}logKeyFound(key, propertySource, value);return convertValueIfNecessary(value, targetValueType);}}}if (logger.isTraceEnabled()) {logger.trace("Could not find key '" + key + "' in any property source");}return null;
}

解决方式

避免存在与系统或环境变量有同名的配置

myname=admin
password=pass

错乱的注入集合

问题

假设存在多个学生Bean，需找出来并存储到List里并在控制器类中输出：

// 可以理解为收集方式
@Bean
public Student student1(){return createStudent(1, "psj1");
}@Bean
public Student student2(){return createStudent(2, "psj2");
}private Student createStudent(int id, String name) {Student student = new Student();student.setId(id);student.setName(name);return student;
}

private List<Student> students;public StudentController(List<Student> students){this.students = students;
}@RequestMapping(path = "students", method = RequestMethod.GET)
public String listStudents(){return students.toString();
};

此时需要再增加学生Bean，换了一种方式注入集合类型：

// 可以理解为直接装配方式
@Bean
public List<Student> students(){Student student3 = createStudent(3, "psj3");Student student4 = createStudent(4, "psj4");return Arrays.asList(student3, student4);
}

但上述两种方式都存在，只会输出前面两个学生

原因

• 进行第一种装配方式时(即收集方式)，主要分为以下过程：

// DefaultListableBeanFactory#resolveMultipleBeans
@Nullable
private Object resolveMultipleBeans(DependencyDescriptor descriptor, @Nullable String beanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) {final Class<?> type = descriptor.getDependencyType();if (descriptor instanceof StreamDependencyDescriptor) {...// 装配streamreturn stream;}else if (type.isArray()) {...// 装配数组return result;}else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {// 装配集合// 获取集合的元素类型Class<?> elementType = descriptor.getResolvableType().asCollection().resolveGeneric();if (elementType == null) {return null;}// 根据元素类型查找所有的beanMap<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType,new MultiElementDescriptor(descriptor));if (matchingBeans.isEmpty()) {return null;}if (autowiredBeanNames != null) {autowiredBeanNames.addAll(matchingBeans.keySet());}// 转化查到的所有bean放置到集合并返回TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());Object result = converter.convertIfNecessary(matchingBeans.values(), type);...return result;}else if (Map.class == type) {...// 解析mapreturn matchingBeans;}else {return null;}
}

• 进行第二种装配方式时(即直接装配方式)，具体过程在DefaultListableBeanFactory#findAutowireCandidates中
• 当同时满足这两种装配方式时，从下面代码中可以看出它们是不能共存的：

// DefaultListableBeanFactory#doResolveDependency
// 采用收集方式
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {return multipleBeans;
}
// 上述方式不执行才会执行直接装配方式
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);

解决方式

对于同一个集合对象的注入不要混合多种注入方式

参考

极客时间-Spring 编程常见错误 50 例

https://github.com/jiafu1115/springissue/tree/master/src/main/java/com/spring/puzzle/class3