Spring boot如何自动加载
对于Springboot的ConditionalOnClass注解一直非常好奇，原因是我们的jar包里面可能没有对应的class，而使用ConditionalOnClass标注的Configuration类又import了这个类，那么如果想加载Configuration类，就会报ClassNotFoundException,那么又如何取到这个类上的注解呢
SpringFactoriesLoader获取"META-INF/spring.factories"路径下的所有文件，解析出需要自动加载的类
判断的逻辑为配置中的org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxx
xxx即为我们配置的需要自动加载的@Configuration标注的类
解析出需要加载的所有Configuration类，无论该类是否被ConditionOnClass注解声明，都使用OnClassCondition类进行match,判断是否需要加载当前类
做这个解析有点耗时，spring boot将筛选出了所有Configuration类数目的一半，单独放到另外一个线程中执行，这样相当于并发两个线程解析
可以参照OnClassCondition类的getOutcomes方法
具体执行解析操作的类为StandardOutcomesResolver，方法：resolveOutcomes()
以Gson自动配置类org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration 为例
package org.springframework.boot.autoconfigure.gson;
import com.google.gson.Gson;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
/**
* {@link EnableAutoConfiguration Auto-configuration} for Gson.
*
* @author David Liu
* @since 1.2.0
*/
@Configuration
@ConditionalOnClass(Gson.class)
public class GsonAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public Gson gson() {
return new Gson();
}
}
假如当前classpath下并没有引入Gson类的jar包
private ConditionOutcome[] getOutcomes(final String[] autoConfigurationClasses,
int start, int end, AutoConfigurationMetadata autoConfigurationMetadata) {
ConditionOutcome[] outcomes = new ConditionOutcome[end - start];
for (int i = start; i < end; i++) {
String autoConfigurationClass = autoConfigurationClasses[i]; //GsonAutoConfiguration类的字符串
Set<String> candidates = autoConfigurationMetadata
.getSet(autoConfigurationClass, "ConditionalOnClass"); //获取当前class上的ConditionOnClass注解配置
if (candidates != null) {
outcomes[i - start] = getOutcome(candidates);
}
}
return outcomes;
}
AutoConfigurationMetadataLoader类将加载META-INF/spring-autoconfigure-metadata.properties下所有的配置，如果你使用了ConditionalOnClass注解，需要写到文件中，如
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration.ConditionalOnClass=io.searchbox.client.JestClient
这样Set
下面的速度会明显变慢，原因是将根据上述执行结果，找出对应的Class是否在classpath*下可以找到
private ConditionOutcome getOutcome(Set<String> candidates) {
try {
List<String> missing = getMatches(candidates, MatchType.MISSING,
this.beanClassLoader);
if (!missing.isEmpty()) {
return ConditionOutcome.noMatch(
ConditionMessage.forCondition(ConditionalOnClass.class)
.didNotFind("required class", "required classes")
.items(Style.QUOTE, missing));
}
}
catch (Exception ex) {
// We'll get another chance later
}
return null;
}
}
使用MatchType.MISSING来判断，如果不为空，则说明缺少这个类了。
private enum MatchType {
PRESENT {
@Override
public boolean matches(String className, ClassLoader classLoader) {
return isPresent(className, classLoader);
}
},
MISSING {
@Override
public boolean matches(String className, ClassLoader classLoader) {
return !isPresent(className, classLoader);
}
};
private static boolean isPresent(String className, ClassLoader classLoader) {
if (classLoader == null) {
classLoader = ClassUtils.getDefaultClassLoader();
}
try {
forName(className, classLoader);
return true;
}
catch (Throwable ex) {
return false;
}
}
private static Class<?> forName(String className, ClassLoader classLoader)
throws ClassNotFoundException {
if (classLoader != null) {
return classLoader.loadClass(className);
}
return Class.forName(className);
}
public abstract boolean matches(String className, ClassLoader classLoader);
}
最终回到了原始的方法，调用classLoader.loadClass(className)来判断类是否在classpath下，加载类相对于内存计算，比较耗时，这也是为什么需要再开一个线程和主线程一起工作的原因,使用Thread.join()来等待线程结束，并获取最终结果
延伸
既然加载ConfigurationBean时，用ClassNotFound就能发现对应的类没有在classpath下，又何必多此一举，绕这么大个弯来发现没有对应的class呢？
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原因是ConditionalOnClass还支持输入字符串类型的class name，在Configuration中可以面向接口编程的方式来生成bean
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Spring boot还提供了类似ConditionalOnBean的注解，有可能一个class在classpath下，而不是spring里面的bean；
@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 {};}