学习 Spring 代码，最重要的是掌握 Spring 有哪些扩展点，可以利用这些扩展点对 Spring 做什么扩展操作。说得更具体一点，如果自己开发一个框架，如何与 Spring 进行整合，如果对 Spring 的扩展点有一个比较清晰的认识，势必会事半功倍。
@Import 先来看一下 @Import 注解的定义：
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented public @interface Import { /** * {@link Configuration @Configuration}, {@link ImportSelector}, * {@link ImportBeanDefinitionRegistrar}, or regular component classes to import. */ Class<?>[] value(); } 从声明可以看出，使用时，只需要指定 Class 实例即可；从方法的文档中可以看出，Class 实例可以分为三种：ImportSelector、ImportBeanDefinitionRegistrar 和常规组件类。示例如下：
@Configuration @Import(LogImportSelector.class) public static class Config { } 在 org.springframework.context.annotation.ConfigurationClassParser#processImports 方法中，集中了对 @Import 注解的处理。从代码可以非常清晰地看出，分了三种情况进行处理：
ImportSelector
ImportBeanDefinitionRegistrar
常规组件 Class
下面分别对其进行介绍。
ImportSelector 先来看一下 ImportSelector 接口的定义：
/** * Interface to be implemented by types that determine which @{@link Configuration} * class(es) should be imported based on a given selection criteria, usually one or * more annotation attributes. * * <p>{@code ImportSelector} implementations are usually processed in the same way * as regular {@code @Import} annotations, however, it is also possible to defer * selection of imports until all {@code @Configuration} classes have been processed * (see {@link DeferredImportSelector} for details). * * @since 3.1 * @see DeferredImportSelector * @see Import * @see ImportBeanDefinitionRegistrar * @see Configuration */ public interface ImportSelector { /** * Select and return the names of which class(es) should be imported based on * the {@link AnnotationMetadata} of the importing @{@link Configuration} class. * @return the class names, or an empty array if none */ String[] selectImports(AnnotationMetadata importingClassMetadata); /** * Return a predicate for excluding classes from the import candidates, to be * transitively applied to all classes found through this selector's imports. * <p>If this predicate returns {@code true} for a given fully-qualified * class name, said class will not be considered as an imported configuration * class, bypassing class file loading as well as metadata introspection. * @return the filter predicate for fully-qualified candidate class names * of transitively imported configuration classes, or {@code null} if none * @since 5.2.4 */ @Nullable default Predicate<String> getExclusionFilter() { return null; } }
以前无意间搜资料了解到 HikariCP，一下子就被它的简洁代码和卓越性能吸引住了。以前也有翻过它的代码，但是不是很系统，最近再次翻阅，正好做些笔记，方便以后学习。
D瓜哥最近在学习 Java 并发知识。那就从 HikariCP 自定义的并发集合 ConcurrentBag 开始学习。
在 HikariCP 的 Wiki 中，有 Down the Rabbit Hole · ConcurrentBag 的章节来专门介绍 ConcurrentBag：
ConcurrentBag 的灵感借鉴自 C# .NET 的 ConcurrentBag 类。但是实现却是完全不同的。这里的 ConcurrentBag 有如下特性：
A lock-free design
ThreadLocal caching
Queue-stealing
Direct hand-off optimizations
下面，通过代码来对此做个说明。
在 ConcurrentBag 类的定义中，声明了集合元素必须是 IConcurrentBagEntry 的子类。先来看看这个接口的定义：
public interface IConcurrentBagEntry { int STATE_NOT_IN_USE = 0; int STATE_IN_USE = 1; int STATE_REMOVED = -1; int STATE_RESERVED = -2; boolean compareAndSet(int expectState, int newState); void setState(int newState); int getState(); } 接下来，看一下成员变量：
// 存放共享元素 private final CopyOnWriteArrayList<T> sharedList; private final boolean weakThreadLocals; // 在 ThreadLocal 缓存线程本地元素，避免线程争用 private final ThreadLocal<List<Object>> threadList; private final IBagStateListener listener; // private final AtomicInteger waiters; private volatile boolean closed; // 接力队列 private final SynchronousQueue<T> handoffQueue; 在 ConcurrentBag 开头的 JavaDoc 中就做了明确说明：
Note that items that are "borrowed" from the bag are not actually removed from any collection, so garbage collection will not occur even if the reference is abandoned. Thus care must be taken to "requite" borrowed objects otherwise a memory leak will result. Only the "remove" method can completely remove an object from the bag.
