spring-core中@Order和Ordered接口的源碼解析說明

spring版本為5.0.11

注意：注解@Order或者接口Ordered的作用是定義Spring IOC容器中Bean的執行順序的優先級，而不是定義Bean的加載順序，Bean的加載順序不受@Order或Ordered接口的影響；

Ordered接口，用來排序的。

Spring是一個大量使用策略設計模式的框架，這意味着有很多相同接口的實現類，那么必定會有優先級的問題。

於是，Spring就提供了Ordered這個接口，來處理相同接口實現類的優先級問題。

getOrder()值越小，優先級越高。

PriorityOrdered是個接口，繼承自Ordered接口，未定義任何方法

     OrderComparator比較器進行排序的時候，若2個對象中有一個對象實現了PriorityOrdered接口，那么這個對象的優先級更高。

　　若2個對象都是PriorityOrdered或Ordered接口的實現類，那么比較Ordered接口的getOrder方法得到order值，值越低，優先級越高。

@Order

1.spring4.2可以用@Order控制配置類的加載順序

2.常用於aop優先事務執行

3.@Order不標注數字，默認最低優先級，即int的最大值

Ordered接口的源碼

/**
 * {@code Ordered} is an interface that can be implemented by objects that
 * should be <em>orderable</em>, for example in a {@code Collection}.
 *
 * <p>The actual {@link #getOrder() order} can be interpreted as prioritization,
 * with the first object (with the lowest order value) having the highest
 * priority.
 *
 * <p>Note that there is also a <em>priority</em> marker for this interface:
 * {@link PriorityOrdered}. Consult the Javadoc for {@code PriorityOrdered} for
 * details on how {@code PriorityOrdered} objects are ordered relative to
 * <em>plain</em> {@link Ordered} objects.
 *
 * <p>Consult the Javadoc for {@link OrderComparator} for details on the
 * sort semantics for non-ordered objects.
 *
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @since 07.04.2003
 * @see PriorityOrdered
 * @see OrderComparator
 * @see org.springframework.core.annotation.Order
 * @see org.springframework.core.annotation.AnnotationAwareOrderComparator
 */
public interface Ordered {

    /**
     * Useful constant for the highest precedence value.
     * @see java.lang.Integer#MIN_VALUE
     */
    int HIGHEST_PRECEDENCE = Integer.MIN_VALUE;

    /**
     * Useful constant for the lowest precedence value.
     * @see java.lang.Integer#MAX_VALUE
     */
    int LOWEST_PRECEDENCE = Integer.MAX_VALUE;


    /**
     * Get the order value of this object.
     * <p>Higher values are interpreted as lower priority. As a consequence,
     * the object with the lowest value has the highest priority (somewhat
     * analogous to Servlet {@code load-on-startup} values).
     * <p>Same order values will result in arbitrary sort positions for the
     * affected objects.
     * @return the order value
     * @see #HIGHEST_PRECEDENCE
     * @see #LOWEST_PRECEDENCE
     */
    int getOrder();

}

@Order源碼

/**
 * {@code @Order} defines the sort order for an annotated component.
 *
 * <p>The {@link #value} is optional and represents an order value as defined in the
 * {@link Ordered} interface. Lower values have higher priority. The default value is
 * {@code Ordered.LOWEST_PRECEDENCE}, indicating lowest priority (losing to any other
 * specified order value).
 *
 * <p><b>NOTE:</b> Since Spring 4.0, annotation-based ordering is supported for many
 * kinds of components in Spring, even for collection injection where the order values
 * of the target components are taken into account (either from their target class or
 * from their {@code @Bean} method). While such order values may influence priorities
 * at injection points, please be aware that they do not influence singleton startup
 * order which is an orthogonal concern determined by dependency relationships and
 * {@code @DependsOn} declarations (influencing a runtime-determined dependency graph).
 *
 * <p>Since Spring 4.1, the standard {@link javax.annotation.Priority} annotation
 * can be used as a drop-in replacement for this annotation in ordering scenarios.
 * Note that {@code @Priority} may have additional semantics when a single element
 * has to be picked (see {@link AnnotationAwareOrderComparator#getPriority}).
 *
 * <p>Alternatively, order values may also be determined on a per-instance basis
 * through the {@link Ordered} interface, allowing for configuration-determined
 * instance values instead of hard-coded values attached to a particular class.
 *
 * <p>Consult the javadoc for {@link org.springframework.core.OrderComparator
 * OrderComparator} for details on the sort semantics for non-ordered objects.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 2.0
 * @see org.springframework.core.Ordered
 * @see AnnotationAwareOrderComparator
 * @see OrderUtils
 * @see javax.annotation.Priority
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Documented
public @interface Order {

    /**
     * The order value.
     * <p>Default is {@link Ordered#LOWEST_PRECEDENCE}.
     * @see Ordered#getOrder()
     */
    int value() default Ordered.LOWEST_PRECEDENCE;

}