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一、前言
- Springboot 源码解析是一件大工程，逐行逐句的去钻研代码，会很干燥，也不容易坚持下去。
- 咱们不谋求大而全，而是试着每次去钻研一个小知识点，最终聚沙成塔，这就是咱们的 springboot 源码管中窥豹系列。
二、排序
- 前几节咱们讲源码的时候，会遇到一些排序的问题，咱们都避而不谈
比方获取 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;
}
比方对 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 外面
runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
// (2) 找到 CommandLineRunner 的实现类，加到 list 外面
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);
}
}
}咱们来剖析一下这个排序 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);
}
}- 咱们晓得排序要么实现 Comparable 接口，要么 new Comparator, spring 用的第二种
- 咱们重点看下这个 comparator : AnnotationAwareOrderComparator
public class AnnotationAwareOrderComparator extends OrderComparator {...}
public class OrderComparator implements Comparator<Object> {...}
compare 办法在 OrderComparator 外面：
@Override
public 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，咱们能够间接看最初一行。
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
- 留神，order 值越小，优先级越高
- 留神，下面 findOrder 办法是 protected, 咱们最开始说的 AnnotationAwareOrderComparator 对它进行了重写
@Override
@Nullable
protected Integer findOrder(Object obj) {Integer order = super.findOrder(obj);
if (order != null) {return order;}
return findOrderFromAnnotation(obj);
}
@Nullable
private 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 办法
- 没找到，再调用 findOrderFromAnnotation 办法
- 最重要的是这一行：OrderUtils.getOrderFromAnnotations(element, annotations)，咱们进去看看
@Nullable
static 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;
}
@Nullable
private 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";- 两个办法，第一个是简略的缓存，不必看，重点看第二个办法
- 先判断有没有 org.springframework.core.annotation.Order 注解
- 再判断有没有 javax.annotation.Priority 注解
- 有注解就去下面的值，没有返回 null
回到最开始的办法，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，咱们的排序源码就剖析完了，咱们总结一下：
- （1）先判断有没有实现 PriorityOrdered，实现 PriorityOrdered 比没实现的有高优先级
- （2）第一步如果比拟不进去，判断有没有实现 Ordered，如果实现了，取实现办法的 int 值比拟
- （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，进行比拟
- （4）如果没有 Order 注解，判断有没有 javax.annotation.Priority 注解，取注解上的值比拟
- （5）如果下面都没有，返回 Integer.MAX_VALUE，值越大优先级越低
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