SpringBoot中ConditionalOnClass注解的原理

SpringBoot中的自动配置类有很多ConditionalOnClass注解，@ConditionalOnClass 在注解值中所有的类都存在时（通过尝试使用类加载器加载指定的类的方式判断）才会匹配，

那这些ConditionalOnClass注解的原理是什么呢，了解ConditionalOnClass注解的原理前要先了解Conditional注解的原理，因为Conditional注解是最基础的

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

Contional注解里可以标注的属性是一个Class数组

而处理这个注解的是一个Condition接口，SpringBootCondition就实现了这个接口来对Contional注解进行处理

@FunctionalInterface
public interface Condition {
    boolean matches(ConditionContext var1, AnnotatedTypeMetadata var2);
}

public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
    String classOrMethodName = getClassOrMethodName(metadata);

    try {
        //获取ContionOutCome对象，这个对象的属性有是否匹配和匹配信息
        ConditionOutcome outcome = this.getMatchOutcome(context, metadata);//getMatchOutcome方法在本类是一个抽象方法，具体实现是在子类实现的，OnClassCondition就实现了这个方法
        this.logOutcome(classOrMethodName, outcome);
        this.recordEvaluation(context, classOrMethodName, outcome);
        return outcome.isMatch();//返回匹配结果 true或者是False
    } 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);
    }
}

了解完Conditional注解的原理之后就可以来了解ConditionalOnClass注解的原理了，可以先看下ConditionalOnClass注解的定义

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

    String[] name() default {};
}

可以看到ConditionalOnClass注解其实是通过Conditional注解起作用的，Conditional注解标注的属性是OnClassCondition.class，接着来看OnClassCondition.class的源码，从源码可以看到OnClassCondition.class是通过getMatchOutCome来获取匹配结果的，

而匹配结果是在SpringBootCondition里被使用的

public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
          //获取容器的类加载器
        ClassLoader classLoader = context.getClassLoader();
        ConditionMessage matchMessage = ConditionMessage.empty();
    // 获取@ConditionalOnClass注解 value以及name属性声明的所有类
        List<String> onClasses = this.getCandidates(metadata, ConditionalOnClass.class);
        List onMissingClasses;
        if (onClasses != null) {
            //加载ConditionalOnClass注解指定的类
            onMissingClasses = this.filter(onClasses, ClassNameFilter.MISSING, classLoader);
            // 如果加载成功即类路径上有ConditionalOnClasses注解指定的类，也就是说onMissingClasses为空，加载失败即onMissingClasses不为空，返回一个匹配失败的结果
            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));
        }
          
        
        onMissingClasses = this.getCandidates(metadata, ConditionalOnMissingClass.class);
        if (onMissingClasses != null) {
            
            //加载ConditionalOnMissingClass注解指定的类
            List<String> present = this.filter(onMissingClasses, ClassNameFilter.PRESENT, classLoader);
            // 如果加载失败即类路径上没有ConditionalOnMissingClasses注解指定的类，也就是说present为空，加载成功即present不为空，返回一个匹配失败的结果
            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);//返回成功匹配的结果
    }

public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
    String classOrMethodName = getClassOrMethodName(metadata);

    try {
        ConditionOutcome outcome = this.getMatchOutcome(context, metadata);//上面方法返回的匹配结果是在这里使用的
        this.logOutcome(classOrMethodName, outcome);
        this.recordEvaluation(context, classOrMethodName, outcome);
        return outcome.isMatch();//匹配成功则返回true，把ConditionalOnClass标记的类加入容器中,匹配失败则跳过标注的类
    } 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);
    }
}

可以看出getMatchOutCome方法里是通过filter方法来判断类路径上是否有ConditionalOnClass注解里标注的类

protected final List<String> filter(Collection<String> classNames, FilteringSpringBootCondition.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();
                //classNameFilter为MISSING时，如果加载到了 
                //ConditionalOnClass指定的类，则matches的返回值为flase，则不会把ConditionalOnClass注解的属性加入到matches，即matches为空
                if (classNameFilter.matches(candidate, classLoader)) {
                    matches.add(candidate);
                }
            }

            return matches;
        }
    }

filter方法是通过ClassNameFilter这个枚举类里的matches方法来判断类路径上是否有ConditionalOnClass注解里标注的类

protected static enum ClassNameFilter {
    //返回类路径中是否存在该类,该类能被加载则返回True，否则返回False
        PRESENT {
            public boolean matches(String className, ClassLoader classLoader) {
                return isPresent(className, classLoader);
            }
        },
    //返回类路径中是否不存在该类,该类能被加载则返回False，否则返回True
        MISSING {
            public boolean matches(String className, ClassLoader classLoader) {
                return !isPresent(className, classLoader);
            }
        };

        private ClassNameFilter() {
        }

        abstract boolean matches(String var1, ClassLoader var2);

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

            try {
                //利用loadClass以及forName方法，判断类路径下有没有这个指定的类
                //有则返回true
                FilteringSpringBootCondition.resolve(className, classLoader);
                return true;
            } catch (Throwable var3) {
                //没有则加载失败返回false
                return false;
            }
        }
    }
}

protected static Class<?> resolve(String className, ClassLoader classLoader) throws ClassNotFoundException {
    return classLoader != null ? classLoader.loadClass(className) : Class.forName(className);//有类加载器就通过类加载器加载，没有则通过forName方法加载
}

看到这里我们就可以知道ConditionalOnClass注解里的类属性是通过类加载器和forName的方法判断类路径上是否有该类，如果类路径上有该类则加载成功，也就是能够成功匹配，成功匹配后SpringBoot就会把ConditionalOnClass注解标记的类加入到容器中