spring boot插件开发实战和原理

>版权声明：本文为博主原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接和本声明。

本文链接：[https://gudepeng.github.io/note/2020/03/22/sprngfactories/](https://gudepeng.github.io/note/2020/03/22/sprngfactories/)

 

## 一.实战：编写spring boot插件

### 1.为什么要编写boot插件

因为我们在开发的时候需要提供一些共同的功能，所以我们编写个共同的jar包。开发人员在使用jar包的时候不用考虑jar包的内容，直接使用具体的功能即可，但是可能由于包路径的不同，你所编写的bean没有被初始化到spring容器中。不应该让开发人员去扫描你的包路径去初始化bean。所以我们要自己动手去把bean初始化到bean容器中，这也是spring扩展能力的由来（spriing.factories）

 

### 2.实战

编写插件代码,编写配置类（例如：DemoAutoConfig），在其中定义你需要的bean

在resources下创建META-INF/spring.factories

编写spring.factories

```

org.springframework.boot.autoconfigure.EnableAutoConfiguration=

com.demo.DemoAutoConfig

```

 

## 二.spring.factories 常用配置接口

### 1. org.springframework.boot.SpringApplicationRunListener

SpringApplicationRunListener来监听Spring Boot的启动流程，并且在各个流程中处理自己的逻辑。在应用启动时，在Spring容器初始化的各个阶段回调对应的方法。

 

### 2. org.springframework.context.ApplicationContextInitializer

ApplicationContextInitializer是在springboot启动过程上下文 ConfigurableApplicationContext刷新方法前(refresh)调用，对ConfigurableApplicationContext的实例做进一步的设置或者处理。

 

### 3.org.springframework.boot.autoconfigure.EnableAutoConfiguration

定义系统自动装配的类。

 

### 4.org.springframework.boot.env.EnvironmentPostProcessor

配置环境的集中管理。比如扩展去做排除加载系统默认的哪些配置类，方便自定义扩展。

 

## 三.spring factories 原理

### 1.获取配置流程

在启动类注解@SpringBootApplication中可以看到引用了@EnableAutoConfiguration。

其中@Import(AutoConfigurationImportSelector.class)

```

public String[] selectImports(AnnotationMetadata annotationMetadata) {

if (!isEnabled(annotationMetadata)) {

return NO_IMPORTS;

}

AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader

.loadMetadata(this.beanClassLoader);

AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,

annotationMetadata);

return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

}

```

其中getAutoConfigurationEntry方法

```

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,

AnnotationMetadata annotationMetadata) {

if (!isEnabled(annotationMetadata)) {

return EMPTY_ENTRY;

}

AnnotationAttributes attributes = getAttributes(annotationMetadata);

List configurations = getCandidateConfigurations(annotationMetadata, attributes);

configurations = removeDuplicates(configurations);

Set exclusions = getExclusions(annotationMetadata, attributes);

checkExcludedClasses(configurations, exclusions);

configurations.removeAll(exclusions);

configurations = filter(configurations, autoConfigurationMetadata);

fireAutoConfigurationImportEvents(configurations, exclusions);

return new AutoConfigurationEntry(configurations, exclusions);

}

```

其中getCandidateConfigurations

```

protected List getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {

List configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),

getBeanClassLoader());

Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "

+ "are using a custom packaging, make sure that file is correct.");

return configurations;

}

```

调用了SpringFactoriesLoader.loadFactoryNames

```

public static List loadFactoryNames(Class factoryType, @Nullable ClassLoader classLoader) {

String factoryTypeName = factoryType.getName();

return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());

}

```

loadSpringFactories方法

```

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

 

private static final Map> cache = new ConcurrentReferenceHashMap<>();

 

private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) {

MultiValueMap result = cache.get(classLoader);

if (result != null) {

return result;

}

 

try {

Enumeration urls = (classLoader != null ?

classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :

ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

result = new LinkedMultiValueMap<>();

while (urls.hasMoreElements()) {

URL url = urls.nextElement();

UrlResource resource = new UrlResource(url);

Properties properties = PropertiesLoaderUtils.loadProperties(resource);

for (Map.Entry entry : properties.entrySet()) {

String factoryTypeName = ((String) entry.getKey()).trim();

for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {

result.add(factoryTypeName, factoryImplementationName.trim());

}

}

}

cache.put(classLoader, result);

return result;

}

catch (IOException ex) {

throw new IllegalArgumentException("Unable to load factories from location [" +

FACTORIES_RESOURCE_LOCATION + "]", ex);

}

}

```

### 2.加载配置流程

在main方法启动的时候我们会调用SpringApplication.run方法

run方法中调用了getSpringFactoriesInstances

调用createSpringFactoriesInstances

```

private List createSpringFactoriesInstances(Class type, Class[] parameterTypes,

ClassLoader classLoader, Object[] args, Set names) {

List instances = new ArrayList<>(names.size());

for (String name : names) {

try {

Class instanceClass = ClassUtils.forName(name, classLoader);

Assert.isAssignable(type, instanceClass);

Constructor constructor = instanceClass.getDeclaredConstructor(parameterTypes);

T instance = (T) BeanUtils.instantiateClass(constructor, args);

instances.add(instance);

}

catch (Throwable ex) {

throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);

}

}

return instances;

}

```

## 四.总结

这是一种类似插件的设计方式，只要引入对应的jar包，就会扫描到jar里的spring.factories，对应的实现类也就会被实例化。