一、前言

Springboot源码解析是一件大工程，逐行逐句的去研究代码，会很枯燥，也不容易坚持下去。
我们不追求大而全，而是试着每次去研究一个小知识点，最终聚沙成塔，这就是我们的springboot源码管中窥豹系列。

二、排序

前几节我们讲源码的时候，会遇到一些排序的问题，我们都避而不谈

比如获取initializer时的排序：

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {ClassLoader classLoader = getClassLoader();Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);// 排序AnnotationAwareOrderComparator.sort(instances);return instances;}

比如对runner排序：

private void callRunners(ApplicationContext context, ApplicationArguments args) {List<Object> runners = new ArrayList<>();// (1) 找到ApplicationRunner的实现类，加到list里面runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());// (2) 找到CommandLineRunner的实现类，加到list里面runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());// (3) 排序AnnotationAwareOrderComparator.sort(runners);// (4) 钩子回调for (Object runner : new LinkedHashSet<>(runners)) {if (runner instanceof ApplicationRunner) {callRunner((ApplicationRunner) runner, args);}if (runner instanceof CommandLineRunner) {callRunner((CommandLineRunner) runner, args);}}}

我们来分析一下这个排序AnnotationAwareOrderComparator.sort(list)的源码。

三、源码解析

public static final AnnotationAwareOrderComparator INSTANCE = new AnnotationAwareOrderComparator();public static void sort(List<?> list) {if (list.size() > 1) {list.sort(INSTANCE);}}

已new的AnnotationAwareOrderComparator对象作为参数，排序
list.sort(comparator)是jdk自带的排序, 通用的

default void sort(Comparator<? super E> c) {Object[] a = this.toArray();Arrays.sort(a, (Comparator) c);ListIterator<E> i = this.listIterator();for (Object e : a) {i.next();i.set((E) e);}}

我们知道排序要么实现Comparable接口，要么new Comparator, spring用的第二种
我们重点看下这个comparator : AnnotationAwareOrderComparator

public class AnnotationAwareOrderComparator extends OrderComparator {...}public class OrderComparator implements Comparator<Object> {...}

compare方法在OrderComparator里面：

@Overridepublic int compare(@Nullable Object o1, @Nullable Object o2) {return doCompare(o1, o2, null);}private int doCompare(@Nullable Object o1, @Nullable Object o2, @Nullable OrderSourceProvider sourceProvider) {boolean p1 = (o1 instanceof PriorityOrdered);boolean p2 = (o2 instanceof PriorityOrdered);if (p1 && !p2) {return -1;}else if (p2 && !p1) {return 1;}int i1 = getOrder(o1, sourceProvider);int i2 = getOrder(o2, sourceProvider);return Integer.compare(i1, i2);}

先判断有没有实现PriorityOrdered接口，实现了的比没实现的有高优先级
再用getOrder（）判断

private int getOrder(@Nullable Object obj, @Nullable OrderSourceProvider sourceProvider) {Integer order = null;if (obj != null && sourceProvider != null) {Object orderSource = sourceProvider.getOrderSource(obj);if (orderSource != null) {if (orderSource.getClass().isArray()) {Object[] sources = ObjectUtils.toObjectArray(orderSource);for (Object source : sources) {order = findOrder(source);if (order != null) {break;}}}else {order = findOrder(orderSource);}}}return (order != null ? order : getOrder(obj));}

sourceProvider为空，我们可以直接看最后一行。

int LOWEST_PRECEDENCE = Integer.MAX_VALUE;protected int getOrder(@Nullable Object obj) {if (obj != null) {Integer order = findOrder(obj);if (order != null) {return order;}}return Ordered.LOWEST_PRECEDENCE;}protected Integer findOrder(Object obj) {return (obj instanceof Ordered ? ((Ordered) obj).getOrder() : null);}

判断有没有实现Ordered接口，如果有就取出来它的order值
如果没有，就取Integer.MAX_VALUE
注意，order值越小，优先级越高
注意，上面findOrder方法是protected, 我们最开始说的AnnotationAwareOrderComparator对它进行了重写

@Override@Nullableprotected Integer findOrder(Object obj) {Integer order = super.findOrder(obj);if (order != null) {return order;}return findOrderFromAnnotation(obj);}@Nullableprivate Integer findOrderFromAnnotation(Object obj) {AnnotatedElement element = (obj instanceof AnnotatedElement ? (AnnotatedElement) obj : obj.getClass());MergedAnnotations annotations = MergedAnnotations.from(element, SearchStrategy.TYPE_HIERARCHY);Integer order = OrderUtils.getOrderFromAnnotations(element, annotations);if (order == null && obj instanceof DecoratingProxy) {return findOrderFromAnnotation(((DecoratingProxy) obj).getDecoratedClass());}return order;}

先调用父类的findOrder方法
没找到，再调用findOrderFromAnnotation方法
最重要的是这一行：OrderUtils.getOrderFromAnnotations(element, annotations)，我们进去看看

@Nullablestatic Integer getOrderFromAnnotations(AnnotatedElement element, MergedAnnotations annotations) {if (!(element instanceof Class)) {return findOrder(annotations);}Object cached = orderCache.get(element);if (cached != null) {return (cached instanceof Integer ? (Integer) cached : null);}Integer result = findOrder(annotations);orderCache.put(element, result != null ? result : NOT_ANNOTATED);return result;}@Nullableprivate static Integer findOrder(MergedAnnotations annotations) {MergedAnnotation<Order> orderAnnotation = annotations.get(Order.class);if (orderAnnotation.isPresent()) {return orderAnnotation.getInt(MergedAnnotation.VALUE);}MergedAnnotation<?> priorityAnnotation = annotations.get(JAVAX_PRIORITY_ANNOTATION);if (priorityAnnotation.isPresent()) {return priorityAnnotation.getInt(MergedAnnotation.VALUE);}return null;}private static final String JAVAX_PRIORITY_ANNOTATION = "javax.annotation.Priority";

两个方法，第一个是简单的缓存，不用看，重点看第二个方法
先判断有没有org.springframework.core.annotation.Order注解
再判断有没有javax.annotation.Priority注解
有注解就去上面的值，没有返回null

回到最开始的方法，进行int值的排序

Integer.compare(i1, i2);public static int compare(int x, int y) {return (x < y) ? -1 : ((x == y) ? 0 : 1);}

至此，我们的排序源码就分析完了，我们总结一下：

（1）先判断有没有实现PriorityOrdered，实现PriorityOrdered比没实现的有高优先级
（2）第一步如果比较不出来，判断有没有实现Ordered，如果实现了，取实现方法的int值比较
（3）如果没有实现Ordered，判断有没有org.springframework.core.annotation.Order注解，有注解，取注解上的order值，进行比较
（4）如果没有Order注解，判断有没有javax.annotation.Priority注解，取注解上的值比较
（5）如果上面都没有，返回Integer.MAX_VALUE，值越大优先级越低
目录：springboot源码解析-管中窥豹系列