SpringBoot条件注解原理

文章目录

- SpringBoot条件注解原理
- - [关于Spring Framework中的Conditional](#关于Spring Framework中的Conditional)
  - 关于SpringBootCondition(所有SpringBoot条件注解的根)
  - 关于ConditionalOnClass
  - - 关于OnClassCondition
    - 如果判断某个类不存在,filter方法

SpringBoot封装了很多基于Spring Framework中的Conditional类的实现。

如@ConditionalOnClass，@ConditionalOnBean ...等等

这些注解是从何而来的呢?

关于Spring Framework中的Conditional

java 复制代码

public @interface Conditional {
    Class<? extends Condition>[] value();
}

内部需要一个继承自Condition类的实现来判断是否将该Bean注入到Spring容器中

java 复制代码

public interface Condition {
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

其中matches 方法用于判断是否将该Bean注入到Spring容器中,当matches方法返回true则将该Bean注入到Spring容器中

关于SpringBootCondition(所有SpringBoot条件注解的根)

java 复制代码

public abstract class SpringBootCondition implements Condition {
    
    public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        
        String classOrMethodName = getClassOrMethodName(metadata);
        try {
            ConditionOutcome outcome = this.getMatchOutcome(context, metadata);
            this.logOutcome(classOrMethodName, outcome);
            this.recordEvaluation(context, classOrMethodName, outcome);
            return outcome.isMatch();
        } catch (NoClassDefFoundError var5) {
            throw new IllegalStateException("Could not evaluate condition on " + classOrMethodName + " due to " + var5.getMessage() + " not found. Make sure your own configuration does not rely on that class. This can also happen if you are @ComponentScanning a springframework package (e.g. if you put a @ComponentScan in the default package by mistake)", var5);
        } catch (RuntimeException var6) {
            throw new IllegalStateException("Error processing condition on " + this.getName(metadata), var6);
        }
    }
}

SpringBootCondition实现了Condition接口，并为matches方法定义了一个判断的框架。

其中matches方法如上：

getClassOrMethodName()获取条件注解写在了哪个类或者哪个方法上
ConditionOutcome outcome = this.getMatchOutcome(context, metadata);

该方法用于获取条件的判断结果。其中getMatchOutcome是一个抽象方法，留给子类去实现。

ConditionOutcome中包含boolean的match属性和ConditionMessage message属性。用于记录条件判断的信息。

this.logOutcome(classOrMethodName, outcome); 用于日志记录
this.recordEvaluation(context, classOrMethodName, outcome);

将判断结果记录到ConditionEvalutionReport中

ConditionEvalutionReportLoggingListener会在收到ContextRefreshedEvent事件后把匹配结果用日志打印出来

关于ConditionalOnClass

该注解的作用是当某个特定的类 存在时，该判断为true，如SpringBoot内嵌容器中，如果Tomcat的某个类存在，则默认容器为Tomcat

java 复制代码

@Conditional({OnClassCondition.class})
public @interface ConditionalOnClass {
    Class<?>[] value() default {};

    String[] name() default {};
}

ConditionOnClass是一个组合注解，该注解是由SpringFramework 中Conditional注解标注，所以OnClassCondition必须是继承自Condition类的。

OnClassCondition类定义了该ConditionalOnClass的匹配逻辑。
value属性定义了Class类型的值，是一个数组.如果该Class在项目中存在，则判断为true
name属性Class类的全路径地址，如果ClassLoader加载该类成功，则判断为true

关于OnClassCondition

SpringBootCondition --> FilteringSpringBootCondition --> OnClassCondition

上面分析过SpringBootCondition的匹配逻辑，这里着重看OnClassCondition重载SpringBootCondition的中关于匹配的逻辑方法 ``getMatchOutcome`

java 复制代码

public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        ClassLoader classLoader = context.getClassLoader();
        ConditionMessage matchMessage = ConditionMessage.empty();
        //拿到ConditionalOnClass注解中的value值，要判断是否存在的类名
        List<String> onClasses = this.getCandidates(metadata, ConditionalOnClass.class);
        List onMissingClasses;
        if (onClasses != null) {
        	//判断OnClass中不存在的类
            onMissingClasses = this.filter(onClasses, ClassNameFilter.MISSING, classLoader);
            //如果有类确实，则表示不匹配
            if (!onMissingClasses.isEmpty()) {
                return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnClass.class, new Object[0]).didNotFind("required class", "required classes").items(Style.QUOTE, onMissingClasses));
            }

            matchMessage = matchMessage.andCondition(ConditionalOnClass.class, new Object[0]).found("required class", "required classes").items(Style.QUOTE, this.filter(onClasses, ClassNameFilter.PRESENT, classLoader));
        }

    //下面是ConditionalOnMissingClass的逻辑
        onMissingClasses = this.getCandidates(metadata, ConditionalOnMissingClass.class);
        if (onMissingClasses != null) {
            List<String> present = this.filter(onMissingClasses, ClassNameFilter.PRESENT, classLoader);
            if (!present.isEmpty()) {
                return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnMissingClass.class, new Object[0]).found("unwanted class", "unwanted classes").items(Style.QUOTE, present));
            }

            matchMessage = matchMessage.andCondition(ConditionalOnMissingClass.class, new Object[0]).didNotFind("unwanted class", "unwanted classes").items(Style.QUOTE, this.filter(onMissingClasses, ClassNameFilter.MISSING, classLoader));
        }

        return ConditionOutcome.match(matchMessage);
    }

List onClasses = this.getCandidates(metadata, ConditionalOnClass.class);

拿到ConditionalOnClass注解中的value值，要判断是否存在的类名

onMissingClasses = this.filter(onClasses, ClassNameFilter.MISSING, classLoader);

判断OnClass中不存在的类

如果判断某个类不存在,filter方法

java 复制代码

protected final List<String> filter(Collection<String> classNames, ClassNameFilter classNameFilter, ClassLoader classLoader) {
        if (CollectionUtils.isEmpty(classNames)) {
            return Collections.emptyList();
        } else {
            List<String> matches = new ArrayList(classNames.size());
            Iterator var5 = classNames.iterator();

            while(var5.hasNext()) {
                String candidate = (String)var5.next();
                if (classNameFilter.matches(candidate, classLoader)) {
                    matches.add(candidate);
                }
            }

            return matches;
        }
    }

这里ClassNameFilter传入的是Missing,在ClassNameFilter源码中,主要通过isPresent判断

java 复制代码

	protected static enum ClassNameFilter {
        PRESENT {
            public boolean matches(String className, ClassLoader classLoader) {
                return isPresent(className, classLoader);
            }
        },
        MISSING {
            public boolean matches(String className, ClassLoader classLoader) {
                return !isPresent(className, classLoader);
            }
        };

        private ClassNameFilter() {
        }

        abstract boolean matches(String className, ClassLoader classLoader);

        static boolean isPresent(String className, ClassLoader classLoader) {
            if (classLoader == null) {
                classLoader = ClassUtils.getDefaultClassLoader();
            }

            try {
                
                FilteringSpringBootCondition.resolve(className, classLoader);
                return true;
            } catch (Throwable var3) {
                return false;
            }
        }
    }

resoleve方法如下

java 复制代码

protected static Class<?> resolve(String className, ClassLoader classLoader) throws ClassNotFoundException {
        return classLoader != null ? Class.forName(className, false, classLoader) : Class.forName(className);
}

可以看到isPresent的逻辑是通过FilteringSpringBootCondition.resolve(className, classLoader); 来尝试加载该类，如果能正常加载，则代表该类存在，如果不能则代表该类不存在。