前言
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仿照spring-boot的项目结构以及部分注解，写一个简单的ioc容器。
测试代码完成后，便正式开始这个ioc容器的开发工作。
正文
项目结构
实际上三四个类完全能搞定这个简单的ioc容器，但是出于可扩展性的考虑，还是写了不少的类。
因篇幅限制，接下来只将几个最重要的类的代码贴出来并加以说明，完整的代码请直接参考https://github.com/clayandgithub/simple-ioc。
SimpleAutowired
代码
import java.lang.annotation.*;@Target({ElementType.FIELD})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface SimpleAutowired { boolean required() default true; String value() default ""; // this field is moved from @Qualifier to here for simplicity}说明
@SimpleAutowired的作用是用于注解需要自动装配的字段。
此类和spring的@Autowired的作用类似。但又有以下两个区别：
- @SimpleAutowired只能作用于类字段，而不能作用于方法（这样实现起来相对简单些，不会用到aop）
- @SimpleAutowired中包括了required（是否一定需要装配）和value（要装配的bean的名字）两个字段，实际上是将spring中的@Autowired以及Qualifier的功能简单地融合到了一起
SimpleBean
代码
import java.lang.annotation.*;@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface SimpleBean { String value() default "";}说明
@SimpleBean作用于方法，根据方法返回值来生成一个bean，对应spring中的@Bean
用value来设置要生成的bean的名字
SimpleComponent
代码
import java.lang.annotation.*;@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface SimpleBean { String value() default "";}说明
@SimpleComponent作用于类，ioc容器会为每一个拥有@SimpleComponent的类生成一个bean，对应spring中的@Component。特殊说明，为了简单起见，@SimpleComponent注解的类必须拥有一个无参构造函数，否则无法生成该类的实例，这个在之后的SimpleAppliationContext中的processSingleClass方法中会有说明。
SimpleIocBootApplication
代码
import java.lang.annotation.*;@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface SimpleIocBootApplication { String[] basePackages() default {};}说明
@SimpleIocBootApplication作用于应用的入口类。
这个启动模式是照搬了spring-boot的启动模式，将启动任务委托给SimpleIocApplication来完成。ioc容器将根据注解@SimpleIocBootApplication的相关配置自动扫描相应的package，生成beans并完成自动装配。（如果没有配置，默认扫描入口类（测试程序中的SampleApplication）所在的package及其子package）
以上就是这个ioc容器所提供的所有注解，接下来讲解ioc容器的扫描和装配过程的实现。
SimpleIocApplication
代码
import com.clayoverwind.simpleioc.context.*;import com.clayoverwind.simpleioc.util.LogUtil;import java.util.Arrays;import java.util.Map;import java.util.logging.Logger;public class SimpleIocApplication { private Class<?> applicationEntryClass; private ApplicationContext applicationContext; private final Logger LOGGER = LogUtil.getLogger(this.getClass()); public SimpleIocApplication(Class<?> applicationEntryClass) { this.applicationEntryClass = applicationEntryClass; } public static void run(Class<?> applicationEntryClass, String[] args) { new SimpleIocApplication(applicationEntryClass).run(args); } public void run(String[] args) { LOGGER.info("start running......"); // create application context and application initializer applicationContext = createSimpleApplicationContext(); ApplicationContextInitializer initializer = createSimpleApplicationContextInitializer(applicationEntryClass); // initialize the application context (this is where we create beans) initializer.initialize(applicationContext); // here maybe exist a hidden cast // process those special beans processSpecialBeans(args); LOGGER.info("over!"); } private SimpleApplicationContextInitializer createSimpleApplicationContextInitializer(Class<?> entryClass) { // get base packages SimpleIocBootApplication annotation = entryClass.getDeclaredAnnotation(SimpleIocBootApplication.class); String[] basePackages = annotation.basePackages(); if (basePackages.length == 0) { basePackages = new String[]{entryClass.getPackage().getName()}; } // create context initializer with base packages return new SimpleApplicationContextInitializer(Arrays.asList(basePackages)); } private SimpleApplicationContext createSimpleApplicationContext() { return new SimpleApplicationContext(); } private void processSpecialBeans(String[] args) { callRegisteredRunners(args); } private void callRegisteredRunners(String[] args) { Map<String, SimpleIocApplicationRunner> applicationRunners = applicationContext.getBeansOfType(SimpleIocApplicationRunner.class); try { for (SimpleIocApplicationRunner applicationRunner : applicationRunners.values()) { applicationRunner.run(args); } } catch (Exception e) { throw new RuntimeException(e); } }}说明
前面说到应用的启动会委托SimpleIocApplication来完成，通过将应用入口类（测试程序中的SampleApplication）传入SimpleIocApplication的构造函数，构造出SimpleIocApplication的一个实例并运行run方法。在run方法中，会首先生成一个applicationContext，并调用SimpleApplicationContextInitializer来完成applicationContext的初始化(bean的扫描、装配)。然后调用processSpecialBeans来处理一些特殊的bean，如实现了SimpleIocApplicationRunner接口的bean会调用run方法来完成一些应用程序的启动任务。
这就是这个ioc容器的整个流程。
SimpleApplicationContextInitializer
代码
import java.io.IOException;import java.util.LinkedHashSet;import java.util.List;import java.util.Set;public class SimpleApplicationContextInitializer implements ApplicationContextInitializer<SimpleApplicationContext> { private Set<String> basePackages = new LinkedHashSet<>(); public SimpleApplicationContextInitializer(List<String> basePackages) { this.basePackages.addAll(basePackages); } @Override public void initialize(SimpleApplicationContext applicationContext) { try { applicationContext.scan(basePackages, true); } catch (ClassNotFoundException e) { throw new RuntimeException(e); } catch (IOException e) { throw new RuntimeException(e); } applicationContext.setStartupDate(System.currentTimeMillis()); }}说明
在SimpleIocApplication的run中，会根据basePackages来构造一个SimpleApplicationContextInitializer 的实例，进而通过这个ApplicationContextInitializer来完成SimpleApplicationContext 的初始化。
在SimpleApplicationContextInitializer中， 简单地调用SimpleApplicationContext 中的scan即可完成SimpleApplicationContext的初始化任务
SimpleApplicationContext
说明：
终于到了最重要的部分了，在SimpleApplicationContext中将真正完成扫描、生成bean以及自动装配的任务。这里scan即为SimpleApplicationContext的程序入口，由SimpleApplicationContextInitializer在初始化时调用。
代码的调用逻辑简单易懂，就不多加说明了。
这里只简单列一下各个字段的含义以及几个比较关键的方法的作用。
字段
- startupDate：启动时间记录字段
- scannedPackages：已经扫描的包的集合，保证不重复扫描
- registeredBeans：已经完全装配好并注册好了的bean
- earlyBeans : 只是生成好了，还未装配完成的bean，用于处理循环依赖的问题
- totalBeanCount : 所有bean的计数器，在生成bean的名字时会用到其唯一性
方法
- processEarlyBeans：用于最终装配earlyBeans 中的bean，若装配成功，则将bean移至registeredBeans，否则报错
- scan ： 扫描并处理传入的package集合
- processSingleClass:处理单个类，尝试生成该类的bean并进行装配（前提是此类有@SimpleComponent注解）
- createBeansByMethodsOfClass ： 顾名思义，根据那些被@Bean注解的方法来生成bean
- autowireFields：尝试装配某个bean，lastChance代表是否在装配失败是报错（在第一次装配时，此值为false，在装配失败后会将bean移至earlyBeans，在第二次装配时，此值为true，实际上就是在装配earlyBeans中的bean，因此若仍然装配失败，就会报错）。在这个方法中，装配相应的bean时会从registeredBeans以及earlyBeans中去寻找符合条件的bean，只要找到，不管是来自哪里，都算装配成功。
代码
import com.clayoverwind.simpleioc.context.annotation.SimpleAutowired;import com.clayoverwind.simpleioc.context.annotation.SimpleBean;import com.clayoverwind.simpleioc.context.annotation.SimpleComponent;import com.clayoverwind.simpleioc.context.factory.Bean;import com.clayoverwind.simpleioc.util.ClassUtil;import com.clayoverwind.simpleioc.util.ConcurrentHashSet;import com.clayoverwind.simpleioc.util.LogUtil;import java.io.IOException;import java.lang.annotation.Annotation;import java.lang.reflect.Field;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.util.*;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.atomic.AtomicLong;import java.util.logging.Logger;/** * @author clayoverwind * @E-mail clayanddev@163.com * @version 2017/4/5 */public class SimpleApplicationContext implements ApplicationContext { private long startupDate; private Set<String> scannedPackages = new ConcurrentHashSet<>(); private Map<String, Bean> registeredBeans = new ConcurrentHashMap<>(); private Map<String, Bean> earlyBeans = new ConcurrentHashMap<>(); private final Logger LOGGER = LogUtil.getLogger(this.getClass()); AtomicLong totalBeanCount = new AtomicLong(0L); AtomicLong nameConflictCount = new AtomicLong(0L); @Override public Object getBean(String name) { return registeredBeans.get(name); } @Override public <T> T getBean(String name, Class<T> type) { Bean bean = (Bean)getBean(name); return bean == null ? null : (type.isAssignableFrom(bean.getClazz()) ? type.cast(bean.getObject()) : null); } @Override public <T> T getBean(Class<T> type) { Map<String, T> map = getBeansOfType(type); return map.isEmpty() ? null : type.cast(map.values().toArray()[0]); } @Override public boolean containsBean(String name) { return getBean(name) != null; } @Override public <T> Map<String, T> getBeansOfType(Class<T> type) { Map<String, T> res = new HashMap<>(); registeredBeans.entrySet().stream().filter(entry -> type.isAssignableFrom(entry.getValue().getClazz())).forEach(entry -> res.put(entry.getKey(), type.cast(entry.getValue().getObject()))); return res; } @Override public void setStartupDate(long startupDate) { this.startupDate = startupDate; } @Override public long getStartupDate() { return startupDate; } /** * try to autowire those beans in earlyBeans * if succeed, remove it from earlyBeans and put it into registeredBeans * otherwise ,throw a RuntimeException(in autowireFields) */ private synchronized void processEarlyBeans() { for (Map.Entry<String, Bean> entry : earlyBeans.entrySet()) { Bean myBean = entry.getValue(); try { if (autowireFields(myBean.getObject(), myBean.getClazz(), true)) { registeredBeans.put(entry.getKey(), myBean); earlyBeans.remove(entry.getKey()); } } catch (IllegalAccessException e) { throw new RuntimeException(e); } } } /** * scan base packages and create beans * @param basePackages * @param recursively * @throws ClassNotFoundException */ public void scan(Set<String> basePackages, boolean recursively) throws ClassNotFoundException, IOException { LOGGER.info("start scanning......"); ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); // get all classes who haven't been registered Set<Class<?>> classes = new LinkedHashSet<>(); for (String packageName : basePackages) { if (scannedPackages.add(packageName)) { classes.addAll(ClassUtil.getClassesByPackageName(classLoader, packageName, recursively)); } } // autowire or create bean for each class classes.forEach(this::processSingleClass); processEarlyBeans(); LOGGER.info("scan over!"); } /** * try to create a bean for certain class, put it into registeredBeans if success, otherwise put it into earlyBeans * @param clazz */ private void processSingleClass(Class<?> clazz) { LOGGER.info(String.format("processSingleClass [%s] ...", clazz.getName())); Annotation[] annotations = clazz.getDeclaredAnnotations(); for (Annotation annotation : annotations) { if (annotation instanceof SimpleComponent) { Object instance; try { instance = clazz.newInstance(); } catch (InstantiationException e) { throw new RuntimeException(e); } catch (IllegalAccessException e) { throw new RuntimeException(e); } long beanId = totalBeanCount.getAndIncrement(); SimpleComponent component = (SimpleComponent) annotation; String beanName = component.value(); if (beanName.isEmpty()) { beanName = getUniqueBeanNameByClassAndBeanId(clazz, beanId); } try { if (autowireFields(instance, clazz, false)) { registeredBeans.put(beanName, new Bean(instance, clazz)); } else { earlyBeans.put(beanName, new Bean(instance, clazz)); } } catch (IllegalAccessException e) { throw new RuntimeException(e); } try { createBeansByMethodsOfClass(instance, clazz); } catch (InvocationTargetException e) { throw new RuntimeException(e); } catch (IllegalAccessException e) { throw new RuntimeException(e); } } } } private void createBeansByMethodsOfClass(Object instance, Class<?> clazz) throws InvocationTargetException, IllegalAccessException { List<Method> methods = getMethodsWithAnnotation(clazz, SimpleBean.class); for (Method method : methods) { method.setAccessible(true); Object methodBean = method.invoke(instance); long beanId = totalBeanCount.getAndIncrement(); Class<?> methodBeanClass = methodBean.getClass(); //bean name SimpleBean simpleBean = method.getAnnotation(SimpleBean.class); String beanName = simpleBean.value(); if (beanName.isEmpty()) { beanName = getUniqueBeanNameByClassAndBeanId(clazz, beanId); } // register bean registeredBeans.put(beanName, new Bean(methodBean, methodBeanClass)); } } private List<Method> getMethodsWithAnnotation(Class<?> clazz, Class<?> annotationClass) { List<Method> res = new LinkedList<>(); Method[] methods = clazz.getDeclaredMethods(); for (Method method : methods) { Annotation[] annotations = method.getAnnotations(); for (Annotation annotation : annotations) { if (annotation.annotationType() == annotationClass) { res.add(method); break; } } } return res; } /** * try autowire all fields of a certain instance * @param instance * @param clazz * @param lastChance * @return true if success, otherwise return false or throw a exception if this is the lastChance * @throws IllegalAccessException */ private boolean autowireFields(Object instance, Class<?> clazz, boolean lastChance) throws IllegalAccessException { Field[] fields = clazz.getDeclaredFields(); for (Field field : fields) { Annotation[] annotations = field.getAnnotations(); for (Annotation annotation : annotations) { if (annotation instanceof SimpleAutowired) { SimpleAutowired autowired = (SimpleAutowired) annotation; String beanName = autowired.value(); Bean bean = getSimpleBeanByNameOrType(beanName, field.getType(), true); if (bean == null) { if (lastChance) { if (!autowired.required()) { break; } throw new RuntimeException(String.format("Failed in autowireFields : [%s].[%s]", clazz.getName(), field.getName())); } else { return false; } } field.setAccessible(true); field.set(instance, bean.getObject()); } } } return true; } /** * only used in autowireFields * @param beanName * @param type * @param allowEarlyBean * @return */ private Bean getSimpleBeanByNameOrType(String beanName, Class<?> type, boolean allowEarlyBean) { // 1. by name Bean res = registeredBeans.get(beanName); if (res == null && allowEarlyBean) { res = earlyBeans.get(beanName); } // 2. by type if (type != null) { if (res == null) { res = getSimpleBeanByType(type, registeredBeans); } if (res == null && allowEarlyBean) { res = getSimpleBeanByType(type, earlyBeans); } } return res; } /** * search bean by type in certain beans map * @param type * @param beansMap * @return */ private Bean getSimpleBeanByType(Class<?> type, Map<String, Bean> beansMap) { List<Bean> beans = new LinkedList<>(); beansMap.entrySet().stream().filter(entry -> type.isAssignableFrom(entry.getValue().getClazz())).forEach(entry -> beans.add(entry.getValue())); if (beans.size() > 1) { throw new RuntimeException(String.format("Autowire by type, but more than one instance of type [%s] is founded!", beans.get(0).getClazz().getName())); } return beans.isEmpty() ? null : beans.get(0); } private String getUniqueBeanNameByClassAndBeanId(Class<?> clazz, long beanId) { String beanName = clazz.getName() + "_" + beanId; while (registeredBeans.containsKey(beanName) || earlyBeans.containsKey(beanName)) { beanName = clazz.getName() + "_" + beanId + "_" + nameConflictCount.getAndIncrement(); } return beanName; }}后记
至此，一个简单的ioc容器就完成了，总结一下优缺点。
优点：
小而简单。
可以使用@SimpleBean、@SimpleComponent以及@SimpleAutowired 来完成一些简单但常用的依赖注入任务.
缺点：
很明显，实现过于简单，提供的功能太少。
如果你想了解ioc的实现原理，或者你想要开发一个小型个人项目但又嫌spring过于庞大，这个简单的ioc容器或许可以帮到你。
如果你想做的不仅如此，那么你应该将目光转向spring-boot。
完整代码参考：https://github.com/clayandgithub/simple-ioc。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持武林网。
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