Spring Boot 条件化自动装配
不知道大家有没有遇到过pom文件中加入
spring-boot-starter-jdbc这个jar之后，在不做任何处理的时候，会报如下错误；
Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2019-09-28 19:32:51.878 ERROR 34836 --- [ main] o.s.b.d.LoggingFailureAnalysisReporter :
***************************
APPLICATION FAILED TO START
***************************
Description:
Failed to configure a DataSource: 'url' attribute is not specified and no embedded datasource could be configured.
Reason: Failed to determine a suitable driver class
Action:
Consider the following:
If you want an embedded database (H2, HSQL or Derby), please put it on the classpath.
If you have database settings to be loaded from a particular profile you may need to activate it (no profiles are currently active).
这是因为没有配置数据源的原因导致的，可是问题来了，我只是加了个jar，但是Spring Boot 如何做到加这个jar就必须要配置数据源呢？
我们可以进入org.springframework.boot.autoconfigure.jdbc.DataSourceConfiguration中看下，其中有一段这样的代码
/**
* Hikari DataSource configuration.
*/
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(HikariDataSource.class)
@ConditionalOnMissingBean(DataSource.class)
@ConditionalOnProperty(name = "spring.datasource.type", havingValue = "com.zaxxer.hikari.HikariDataSource",
matchIfMissing = true)
static class Hikari {
@Bean
@ConfigurationProperties(prefix = "spring.datasource.hikari")
HikariDataSource dataSource(DataSourceProperties properties) {
HikariDataSource dataSource = createDataSource(properties, HikariDataSource.class);
if (StringUtils.hasText(properties.getName())) {
dataSource.setPoolName(properties.getName());
}
return dataSource;
}
}
`注：spring boot 默认的是hikari``
原来是通过一些注解做到的；
Class条件注解
@ConditionalOnClass （当指定类存在时）
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {
/**
* The classes that must be present. Since this annotation is parsed by loading class
* bytecode, it is safe to specify classes here that may ultimately not be on the
* classpath, only if this annotation is directly on the affected component and
* <b>not</b> if this annotation is used as a composed, meta-annotation. In order to
* use this annotation as a meta-annotation, only use the {@link #name} attribute.
* @return the classes that must be present
*/
Class<?>[] value() default {};
/**
* The classes names that must be present.
* @return the class names that must be present.
*/
String[] name() default {};
}
@ConditionalOnMissingClass (当指定类缺失时)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnMissingClass {
/**
* The names of the classes that must not be present.
* @return the names of the classes that must not be present
*/
String[] value() default {};
}
注：@ConditionalOnMissingClass注解在spring boot 在各个版本都不同的改动。
Bean条件注解
@ConditionalOnBean (当指定Bean存在)
@ConditionalOnMissingBean（当指定Bean不存在）
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {
/**
* The class types of beans that should be checked. The condition matches when beans
* of all classes specified are contained in the {@link BeanFactory}.
* @return the class types of beans to check
*/
Class<?>[] value() default {};
/**
* The class type names of beans that should be checked. The condition matches when
* beans of all classes specified are contained in the {@link BeanFactory}.
* @return the class type names of beans to check
*/
String[] type() default {};
/**
* The annotation type decorating a bean that should be checked. The condition matches
* when all of the annotations specified are defined on beans in the
* {@link BeanFactory}.
* @return the class-level annotation types to check
*/
Class<? extends Annotation>[] annotation() default {};
/**
* The names of beans to check. The condition matches when all of the bean names
* specified are contained in the {@link BeanFactory}.
* @return the names of beans to check
*/
String[] name() default {};
/**
* Strategy to decide if the application context hierarchy (parent contexts) should be
* considered.
* @return the search strategy
*/
SearchStrategy search() default SearchStrategy.ALL;
/**
* Additional classes that may contain the specified bean types within their generic
* parameters. For example, an annotation declaring {@code value=Name.class} and
* {@code parameterizedContainer=NameRegistration.class} would detect both
* {@code Name} and {@code NameRegistration<Name>}.
* @return the container types
* @since 2.1.0
*/
Class<?>[] parameterizedContainer() default {};
}
- value 类型安全的属性设置
- type 当类型不存在时的属性设置
- annotation 当bean标注了某种注解类型时
- name 指定具体bean名称
- search 三种应用上下文搜索策略；当前、父级所有
- parameterizedContainer 泛型
属性条件注解
@ConditionalOnProperty
String[] value() default {};
// 配置属性名前缀
String prefix() default "";
// 配置属性名，如果prefix不为空则值为 prefix+name
String[] name() default {};
// 表示期望属性的值
String havingValue() default "";
// 用于判断属性值不存在的时候是否匹配
boolean matchIfMissing() default false;
条件注解还有很多，如下
@ConditionalOnClass ： classpath中存在该类时起效
@ConditionalOnMissingClass ： classpath中不存在该类时起效
@ConditionalOnBean ： DI容器中存在该类型Bean时起效
@ConditionalOnMissingBean ： DI容器中不存在该类型Bean时起效
@ConditionalOnSingleCandidate ： DI容器中该类型Bean只有一个或@Primary的只有一个时起效
@ConditionalOnExpression ： SpEL表达式结果为true时
@ConditionalOnProperty ： 参数设置或者值一致时起效
@ConditionalOnResource ： 指定的文件存在时起效
@ConditionalOnJndi ： 指定的JNDI存在时起效
@ConditionalOnJava ： 指定的Java版本存在时起效
@ConditionalOnWebApplication ： Web应用环境下起效
@ConditionalOnNotWebApplication ： 非Web应用环境下起效
具体使用时，可以查看使用方法