ConditionalOnProperty (Spring Boot API)

```
@Retention(RUNTIME)
@Target({TYPE,METHOD})
@Documented
@Conditional(org.springframework.boot.autoconfigure.condition.OnPropertyCondition.class)
public @interface ConditionalOnProperty
```
Conditional that checks if the specified properties have a specific value. By default the properties must be present in the Environment and not equal to false. The havingValue() and matchIfMissing() attributes allow further customizations.
The havingValue() attribute can be used to specify the value that the property should have. The table below shows when a condition matches according to the property value and the havingValue() attribute:
Having values
Property Value havingValue="" havingValue="true" havingValue="false" havingValue="foo"
"true" yes yes no no
"false" no no yes no
"foo" yes no no yes
If the property is not contained in the Environment at all, the matchIfMissing() attribute is consulted. By default missing attributes do not match.
This condition cannot be reliably used for matching collection properties. For example, in the following configuration, the condition matches if spring.example.values is present in the Environment but does not match if spring.example.values[0] is present.
```
 @ConditionalOnProperty(prefix = "spring", name = "example.values")
 class ExampleAutoConfiguration {
 }
 
```
It is better to use a custom condition for such cases.
从以下版本开始:
1.1.0

可选元素概要

可选元素
修饰符和类型	可选元素	说明
`String`	`havingValue`	The string representation of the expected value for the properties.
`boolean`	`matchIfMissing`	Specify if the condition should match if the property is not set.
`String[]`	`name`	The name of the properties to test.
`String`	`prefix`	A prefix that should be applied to each property.
`String[]`	`value`	Alias for `name()`.

- 元素详细资料
  - value
```
String[] value
```
    Alias for name().
    返回:
    the names
    默认值:
    {}
- - prefix
```
String prefix
```
    A prefix that should be applied to each property. The prefix automatically ends with a dot if not specified. A valid prefix is defined by one or more words separated with dots (e.g. "acme.system.feature").
    返回:
    the prefix
    默认值:
    ""
- - name
```
String[] name
```
    The name of the properties to test. If a prefix has been defined, it is applied to compute the full key of each property. For instance if the prefix is app.config and one value is my-value, the full key would be app.config.my-value
    Use the dashed notation to specify each property, that is all lower case with a "-" to separate words (e.g. my-long-property).
    返回:
    the names
    默认值:
    {}
- - havingValue
```
String havingValue
```
    The string representation of the expected value for the properties. If not specified, the property must not be equal to false.
    返回:
    the expected value
    默认值:
    ""
- - matchIfMissing
```
boolean matchIfMissing
```
    Specify if the condition should match if the property is not set. Defaults to false.
    返回:
    if should match if the property is missing
    默认值:
    false