Spring @Value

系统管理员 2024-03-25 19:57 65阅读 0赞

`@Value` can be used for injecting values into fields in Spring-managed beans, and it can be applied at the `field` or `constructor`/`method` parameter level.

## Setting Up the Application ##

We can use placeholder strings in `@Value` to wire values defined in external sources, for example, in `.properties` or `.yaml` files.

# .properties file
    value.from.file=Value got from the file
    priority=high
    listOfValues=A,B,C

## 设置死值 ##

@Value("string value")
    private String stringValue;

## 读配置文件的值 ##

@Value("${value.from.file}")
    private String valueFromFile;

## 读系统变量的值 ##

@Value("${systemValue}")
    private String systemValue;
    
    # 系统变量和配置文件变量重名时，系统变量值优先
    @Value("${priority}")
    private String prioritySystemProperty;

## 批量设置值 ##

@Value("${listOfValues}")
    private String[] valuesArray;

## 可以设置默认值 ##

# String Defaults
    @Value("${some.key:my-default-value}")
    private String stringWithDefaultValue;
    
    # zero-length String as the default value
    @Value("${some.key:})"
    private String stringWithBlankDefaultValue;
    
    # Primitives
    @Value("${some.key:true}")
    private boolean booleanWithDefaultValue;
    
    @Value("${some.key:42}")
    private int intWithDefaultValue;
    
    # Arrays
    @Value("${some.key:one,two,three}")
    private String[] stringArrayWithDefaults;
    
    @Value("${some.key:1,2,3}")
    private int[] intArrayWithDefaults;
    
    #  Using SpEL
    @Value("#{systemProperties['some.key'] ?: 'my default system property value'}")
    private String spelWithDefaultValue;

## Using @Value With Constructor Injection ##

@Component
    @PropertySource("classpath:values.properties")
    public class PriorityProvider {
        
    
        private String priority;
    
        @Autowired
        public PriorityProvider(@Value("${priority:normal}") String priority) {
        
            this.priority = priority;
        }
    
        // standard getter
    }

## Using @Value With Setter Injection ##

@Component
    @PropertySource("classpath:values.properties")
    public class CollectionProvider {
        
    
        private List<String> values = new ArrayList<>();
        private int cylinderCount;
    
        @Autowired
        public void setValues(@Value("#{'${listOfValues}'.split(',')}") List<String> values) {
        
            this.values.addAll(values);
        }
    
        @Autowired
        void setCylinderCount(@Value("8") int cylinderCount) {
        
            this.cylinderCount = cylinderCount;
        }
    
        @Value("8")
        void setCylinderCount(int cylinderCount) {
        
            this.cylinderCount = cylinderCount;
        }
        // standard getter
    }

## Using @Value With SpEL expressions ##

`Spring Expression Language (SpEL)`

#  If we have a system property named priority, then its value will be applied to the field
    #  If we have not defined the system property, then the null value will be assigned.
    @Value("#{systemProperties['priority']}")
    private String spelValue;
    
    # We get some default value for the field if the system property is not defined
    @Value("#{systemProperties['unknown'] ?: 'some default'}")
    private String spelSomeDefault;
    
    # We can manipulate properties to get a List of values
    @Value("#{'${listOfValues}'.split(',')}")
    private List<String> valuesList;

## Using @Value With Maps ##

# First, we'll need to define the property in the {key: ‘value' } form in our properties file.
    # Note that the values in the Map must be in single quotes.
    valuesMap={
        key1: '1', key2: '2', key3: '3'}
    
    @Value("#{${valuesMap}}")
    private Map<String, Integer> valuesMap;
    
    # get the value of a specific key in the Map
    @Value("#{${valuesMap}.key1}")
    private Integer valuesMapKey1;
    
    # If we're not sure whether the Map contains a certain key, 
    # we should choose a safer expression that will not throw an exception but set the value to null when the key is not found:
    @Value("#{${valuesMap}['unknownKey']}")
    private Integer unknownMapKey;
    
    # We can also set default values for the properties or keys that might not exist:
    @Value("#{${unknownMap : {key1: '1', key2: '2'}}}")
    private Map<String, Integer> unknownMap;
    @Value("#{${valuesMap}['unknownKey'] ?: 5}")
    private Integer unknownMapKeyWithDefaultValue;
    
    # 定制条件
    @Value("#{${valuesMap}.?[value>'1']}")
    private Map<String, Integer> valuesMapFiltered;
    
    # inject all current system properties
    @Value("#{systemProperties}")
    private Map<String, String> systemPropertiesMap;

**参考：**  
**[A Quick Guide to Spring @Value][A Quick Guide to Spring _Value]**  
**[Using Spring @Value With Defaults][Using Spring _Value With Defaults]**

[A Quick Guide to Spring _Value]: https://www.baeldung.com/spring-value-annotation
[Using Spring _Value With Defaults]: https://www.baeldung.com/spring-value-defaults