Spring Boot条件化自动装配

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1、Class 条件注解

Class 条件注解有一对语义相反的注解，@ConditionalOnClass 和 @ConditionalOnMissClass 分别表达 ” 当指定类存在时 ” 和 ” 当指定类不存在时 ” 的语义。

源码:

@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 {};}
@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 {};}

2、案例:

如果工程中有 Jackson 时, 使用 JSON 序列化, 没有使用 String.valueOf()格式化

2.1、增加 Jackson 依赖到 pom.xml, 保持 为 true

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.10.3</version>
            <optional>true</optional>
        </dependency>

2.2、新增 JSON Formatter 实现

public class JsonFormatter implements Formatter {private final ObjectMapper objectMapper;
    
    public JsonFormatter(){this.objectMapper=new ObjectMapper();}
		// 将对对象转换为 json 字符串
    public String format(Object obj) {try{return objectMapper.writeValueAsString(obj);
        }catch (JsonProcessingException e){throw new IllegalArgumentException(e);
        }
    }
}

2.3、新增 JsonFormatter Bean 声明到 FormatterAutoConfiguration

@Configuration
public class FormatterAutoConfiguration {@Bean
    @ConditionalOnMissingClass(value = "com.fasterxml.jackson.databind.ObjectMapper")// 如果 classpath 下没有 ObjectMapper 时
    public Formatter defaultFormatter(){return new DefaultFormatter();}

    @Bean
    @ConditionalOnClass(name = "com.fasterxml.jackson.databind.ObjectMapper")// 如果 classpath 下有 ObjectMapper 时
    public Formatter jsonFormatter(){return new JsonFormatter();}
}

2.4、重新构建 formatter-spring-boot-starter

 mvn -Dmaven.test.skip -U clean install

2.5、加入 formatter-spring-boot-starter

        <dependency>
            <groupId>com.tyschool</groupId>
            <artifactId>formatter-spring-boot-starter</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>

3、Bean 条件注解

Bean 条件注解也是成对出现的, 例如 @ConditionalOnBean 和 @ConditionalOnMissBean。ConditionOnBean 仅匹配应用上下文中已处理的 BeanDefinition。ConditionalOnMissBean 逻辑相反。

源码:

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {Class<?>[] value() default {};


	String[] type() default {};


	Class<? extends Annotation>[] annotation() default {};


	String[] name() default {};


	SearchStrategy search() default SearchStrategy.ALL;


	Class<?>[] parameterizedContainer() default {};}
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnMissingBean {Class<?>[] value() default {};


	String[] type() default {};


	Class<?>[] ignored() default {};


	String[] ignoredType() default {};


	Class<? extends Annotation>[] annotation() default {};


	String[] name() default {};


	SearchStrategy search() default SearchStrategy.ALL;


	Class<?>[] parameterizedContainer() default {};}

4、Json 格式化案例

• 当前 ObjectMapper Class 不存在时,Bean 为 DefaultFormatter 实例, 其名称为 ”defaultFormatter”

• 当前 ObjectMapper Class 不存在时且具有 Bean 不存在时,BeanJsonFormatter 默认构造器创建 ObjectMapper 实例, 其名称为 ”jsonFormatter”

• 当 ObjectMapper Class 存在且其 Bean 也存在时,Bean 为 JsonFormatter 构造器注入 ObjectMapperBean, 其名称为 ”objectMapperFormatter”

提示:

ObjectMapper Bean 的初始化需要满足以下条件:

• ObjectMapper 必须存在于 Class Path 中

• Jackson2ObjectMapperBuilder 必须在 Class Path 中, 源于 org.springframework:spring-web 4.1.1, 工程需依赖 spring-boot-strater-web 1.2.0 及以上版本

• ObjectMapper Bean 必须在所有 Spring 应用上下文中

4.1、增加 Jackson2ObjectMapperBuilder Maven 依赖

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
            <version>2.2.6.RELEASE</version>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>2.2.6.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.10.3</version>
            <optional>true</optional>
        </dependency>
    </dependencies>

4.2、增加 JsonFormatter 构造器

    private final ObjectMapper objectMapper;

    public JsonFormatter(){this(new ObjectMapper());
    }
    
    public JsonFormatter(ObjectMapper objectMapper){this.objectMapper=objectMapper;
    }

4.3、新增 JsonFormatterBean

@Configuration
public class FormatterAutoConfiguration {@Bean
    @ConditionalOnMissingClass(value = "com.fasterxml.jackson.databind.ObjectMapper")// 如果 classpath 下没有 ObjectMapper 时
    public Formatter defaultFormatter(){return new DefaultFormatter();}

    @Bean
    @ConditionalOnClass(name = "com.fasterxml.jackson.databind.ObjectMapper")// 如果 classpath 下有 ObjectMapper 时
    @ConditionalOnMissingBean(Servlet.class)
    public Formatter jsonFormatter(){return new JsonFormatter();}

    @Bean
    @ConditionalOnBean(Servlet.class)// 如果 classpath 有 Servlet 时
    public Formatter objectMapperFormatter(ObjectMapper objectMapper){return new JsonFormatter(objectMapper);
    }
}

4.4、重新构建 formatter-spring-boot-starter

 mvn -Dmaven.test.skip -U clean install

4.5、引入 formatter-spring-boot-starter

        <dependency>
            <groupId>com.tyschool</groupId>
            <artifactId>formatter-spring-boot-starter</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency> 

4.6、重构引导类

@SpringBootApplication
public class FormatterBootStrap {public static void main(String[] args) {// 创建 Spring 上下文
        ConfigurableApplicationContext context = new SpringApplicationBuilder(FormatterBootStrap.class)
                .web(WebApplicationType.SERVLET)
                .run(args);
        final Map<String, Object> map = new HashMap<String, Object>();
        map.put("name","tyschool");
        // 获取 bean
        Map<String, Formatter> beans = context.getBeansOfType(Formatter.class);
        // 格式化数据
        beans.forEach((beanName,formatter)->{System.out.println(beanName+""+formatter.getClass().getSimpleName()+" "+formatter.format(map));
        });
        context.close();}
}