本篇我們一起來看一下Spring Framework IoC依賴處理過程解析。這裡我總結了幾個關鍵點：
* 入口 - DefaultListableBeanFactory#resolveDependency
* 依賴描述符 - DependencyDescriptor
* 自綁定候選對象處理器 - AutowireCandidateResolver 接下來我們就通過Debug的方式進行源碼閱讀，一起來了解一下。代碼如下：
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.eleven.thinking.in.spring.ioc.dependency.injection;
import org.eleven.thinking.in.spring.ioc.dependency.injection.annotation.InjectedUser;
import org.eleven.thinking.in.spring.ioc.dependency.injection.annotation.MyAutowired;
import org.eleven.thinking.in.spring.ioc.overview.domain.User;
import org.springframework.beans.factory.ObjectFactory;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.beans.factory.xml.XmlBeanDefinitionReader;
import org.springframework.context.annotation.*;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import javax.inject.Inject;
import java.lang.annotation.Annotation;
import java.util.*;
import static java.util.Arrays.asList;
import static org.springframework.context.annotation.AnnotationConfigUtils.AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME;
/**
* 注解驅動的依賴注入處理過程
*
* @author <a href="mailto:[email protected]">eleven</a>
* @see Qualifier
* @since
*/
@Configuration
public class AnnotationDependencyInjectionResolutionDemo {
@Autowired // 依賴查找（處理） + 延遲
@Lazy
private User lazyUser;
// DependencyDescriptor ->
// 必須（required=true）
// 實時注入（eager=true)
// 通過類型（User.class）
// 字段名稱（"user"）
// 是否首要（primary = true)
@Autowired // 依賴查找（處理）
private User user;
@Autowired // 集合類型依賴注入
private Map<String, User> users; // user superUser
public static void main(String[] args) {
// 建立 BeanFactory 容器
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
// 注冊 Configuration Class（配置類） -> Spring Bean
applicationContext.register(AnnotationDependencyInjectionResolutionDemo.class);
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(applicationContext);
String xmlResourcePath = "classpath:/META-INF/dependency-lookup-context.xml";
// 加載 XML 資源，解析并且生成 BeanDefinition
beanDefinitionReader.loadBeanDefinitions(xmlResourcePath);
// 啟動 Spring 應用上下文
applicationContext.refresh();
// 依賴查找 QualifierAnnotationDependencyInjectionDemo Bean
AnnotationDependencyInjectionResolutionDemo demo = applicationContext.getBean(AnnotationDependencyInjectionResolutionDemo.class);
// 期待輸出 superUser Bean
System.out.println("demo.user = " + demo.user);
// 期待輸出 user superUser
System.out.println("demo.users = " + demo.users);
// 期待輸出 superUser Bean
System.out.println("demo.userOptional = " + demo.userOptional);
// 顯示地關閉 Spring 應用上下文
applicationContext.close();
}
} 以上我們分别掩飾了單一類型、集合類型以及延遲加載的依賴處理過程，大家可以自己實踐一下。另外這裡我們再來看一下DependencyDescriptor這個類的源碼：
/*
* Copyright 2002-2019 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.springframework.beans.factory.config;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.Map;
import java.util.Optional;
import kotlin.reflect.KProperty;
import kotlin.reflect.jvm.ReflectJvmMapping;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.InjectionPoint;
import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
import org.springframework.core.GenericTypeResolver;
import org.springframework.core.KotlinDetector;
import org.springframework.core.MethodParameter;
import org.springframework.core.ParameterNameDiscoverer;
import org.springframework.core.ResolvableType;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.lang.Nullable;
import org.springframework.util.ObjectUtils;
/**
* Descriptor for a specific dependency that is about to be injected.
* Wraps a constructor parameter, a method parameter or a field,
* allowing unified access to their metadata.
*
* @author Juergen Hoeller
* @since 2.5
*/
@SuppressWarnings("serial")
public class DependencyDescriptor extends InjectionPoint implements Serializable {
private final Class<?> declaringClass;
@Nullable
private String methodName;
@Nullable
private Class<?>[] parameterTypes;
private int parameterIndex;
@Nullable
private String fieldName;
private final boolean required;
private final boolean eager;
private int nestingLevel = 1;
@Nullable
private Class<?> containingClass;
@Nullable
private transient volatile ResolvableType resolvableType;
@Nullable
private transient volatile TypeDescriptor typeDescriptor;
/**
* Create a new descriptor for a method or constructor parameter.
* Considers the dependency as 'eager'.
* @param methodParameter the MethodParameter to wrap
* @param required whether the dependency is required
*/
public DependencyDescriptor(MethodParameter methodParameter, boolean required) {
this(methodParameter, required, true);
}
/**
* Create a new descriptor for a method or constructor parameter.
* @param methodParameter the MethodParameter to wrap
* @param required whether the dependency is required
* @param eager whether this dependency is 'eager' in the sense of
* eagerly resolving potential target beans for type matching
*/
public DependencyDescriptor(MethodParameter methodParameter, boolean required, boolean eager) {
super(methodParameter);
this.declaringClass = methodParameter.getDeclaringClass();
if (methodParameter.getMethod() != null) {
this.methodName = methodParameter.getMethod().getName();
}
this.parameterTypes = methodParameter.getExecutable().getParameterTypes();
this.parameterIndex = methodParameter.getParameterIndex();
this.containingClass = methodParameter.getContainingClass();
this.required = required;
this.eager = eager;
}
/**
* Create a new descriptor for a field.
* Considers the dependency as 'eager'.
* @param field the field to wrap
* @param required whether the dependency is required
*/
public DependencyDescriptor(Field field, boolean required) {
this(field, required, true);
}
/**
* Create a new descriptor for a field.
* @param field the field to wrap
* @param required whether the dependency is required
* @param eager whether this dependency is 'eager' in the sense of
* eagerly resolving potential target beans for type matching
*/
public DependencyDescriptor(Field field, boolean required, boolean eager) {
super(field);
this.declaringClass = field.getDeclaringClass();
this.fieldName = field.getName();
this.required = required;
this.eager = eager;
}
/**
* Copy constructor.
* @param original the original descriptor to create a copy from
*/
public DependencyDescriptor(DependencyDescriptor original) {
super(original);
this.declaringClass = original.declaringClass;
this.methodName = original.methodName;
this.parameterTypes = original.parameterTypes;
this.parameterIndex = original.parameterIndex;
this.fieldName = original.fieldName;
this.containingClass = original.containingClass;
this.required = original.required;
this.eager = original.eager;
this.nestingLevel = original.nestingLevel;
}
/**
* Return whether this dependency is required.
* <p>Optional semantics are derived from Java 8's {@link java.util.Optional},
* any variant of a parameter-level {@code Nullable} annotation (such as from
* JSR-305 or the FindBugs set of annotations), or a language-level nullable
* type declaration in Kotlin.
*/
public boolean isRequired() {
if (!this.required) {
return false;
}
if (this.field != null) {
return !(this.field.getType() == Optional.class || hasNullableAnnotation() ||
(KotlinDetector.isKotlinReflectPresent() &&
KotlinDetector.isKotlinType(this.field.getDeclaringClass()) &&
KotlinDelegate.isNullable(this.field)));
}
else {
return !obtainMethodParameter().isOptional();
}
}
/**
* Check whether the underlying field is annotated with any variant of a
* {@code Nullable} annotation, e.g. {@code javax.annotation.Nullable} or
* {@code edu.umd.cs.findbugs.annotations.Nullable}.
*/
private boolean hasNullableAnnotation() {
for (Annotation ann : getAnnotations()) {
if ("Nullable".equals(ann.annotationType().getSimpleName())) {
return true;
}
}
return false;
}
/**
* Return whether this dependency is 'eager' in the sense of
* eagerly resolving potential target beans for type matching.
*/
public boolean isEager() {
return this.eager;
}
/**
* Resolve the specified not-unique scenario: by default,
* throwing a {@link NoUniqueBeanDefinitionException}.
* <p>Subclasses may override this to select one of the instances or
* to opt out with no result at all through returning {@code null}.
* @param type the requested bean type
* @param matchingBeans a map of bean names and corresponding bean
* instances which have been pre-selected for the given type
* (qualifiers etc already applied)
* @return a bean instance to proceed with, or {@code null} for none
* @throws BeansException in case of the not-unique scenario being fatal
* @since 5.1
*/
@Nullable
public Object resolveNotUnique(ResolvableType type, Map<String, Object> matchingBeans) throws BeansException {
throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());
}
/**
* Resolve the specified not-unique scenario: by default,
* throwing a {@link NoUniqueBeanDefinitionException}.
* <p>Subclasses may override this to select one of the instances or
* to opt out with no result at all through returning {@code null}.
* @param type the requested bean type
* @param matchingBeans a map of bean names and corresponding bean
* instances which have been pre-selected for the given type
* (qualifiers etc already applied)
* @return a bean instance to proceed with, or {@code null} for none
* @throws BeansException in case of the not-unique scenario being fatal
* @since 4.3
* @deprecated as of 5.1, in favor of {@link #resolveNotUnique(ResolvableType, Map)}
*/
@Deprecated
@Nullable
public Object resolveNotUnique(Class<?> type, Map<String, Object> matchingBeans) throws BeansException {
throw new NoUniqueBeanDefinitionException(type, matchingBeans.keySet());
}
/**
* Resolve a shortcut for this dependency against the given factory, for example
* taking some pre-resolved information into account.
* <p>The resolution algorithm will first attempt to resolve a shortcut through this
* method before going into the regular type matching algorithm across all beans.
* Subclasses may override this method to improve resolution performance based on
* pre-cached information while still receiving {@link InjectionPoint} exposure etc.
* @param beanFactory the associated factory
* @return the shortcut result if any, or {@code null} if none
* @throws BeansException if the shortcut could not be obtained
* @since 4.3.1
*/
@Nullable
public Object resolveShortcut(BeanFactory beanFactory) throws BeansException {
return null;
}
/**
* Resolve the specified bean name, as a candidate result of the matching
* algorithm for this dependency, to a bean instance from the given factory.
* <p>The default implementation calls {@link BeanFactory#getBean(String)}.
* Subclasses may provide additional arguments or other customizations.
* @param beanName the bean name, as a candidate result for this dependency
* @param requiredType the expected type of the bean (as an assertion)
* @param beanFactory the associated factory
* @return the bean instance (never {@code null})
* @throws BeansException if the bean could not be obtained
* @since 4.3.2
* @see BeanFactory#getBean(String)
*/
public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
throws BeansException {
return beanFactory.getBean(beanName);
}
/**
* Increase this descriptor's nesting level.
* @see MethodParameter#increaseNestingLevel()
*/
public void increaseNestingLevel() {
this.nestingLevel++;
this.resolvableType = null;
if (this.methodParameter != null) {
this.methodParameter.increaseNestingLevel();
}
}
/**
* Optionally set the concrete class that contains this dependency.
* This may differ from the class that declares the parameter/field in that
* it may be a subclass thereof, potentially substituting type variables.
* @since 4.0
*/
public void setContainingClass(Class<?> containingClass) {
this.containingClass = containingClass;
this.resolvableType = null;
if (this.methodParameter != null) {
GenericTypeResolver.resolveParameterType(this.methodParameter, containingClass);
}
}
/**
* Build a {@link ResolvableType} object for the wrapped parameter/field.
* @since 4.0
*/
public ResolvableType getResolvableType() {
ResolvableType resolvableType = this.resolvableType;
if (resolvableType == null) {
resolvableType = (this.field != null ?
ResolvableType.forField(this.field, this.nestingLevel, this.containingClass) :
ResolvableType.forMethodParameter(obtainMethodParameter()));
this.resolvableType = resolvableType;
}
return resolvableType;
}
/**
* Build a {@link TypeDescriptor} object for the wrapped parameter/field.
* @since 5.1.4
*/
public TypeDescriptor getTypeDescriptor() {
TypeDescriptor typeDescriptor = this.typeDescriptor;
if (typeDescriptor == null) {
typeDescriptor = (this.field != null ?
new TypeDescriptor(getResolvableType(), getDependencyType(), getAnnotations()) :
new TypeDescriptor(obtainMethodParameter()));
this.typeDescriptor = typeDescriptor;
}
return typeDescriptor;
}
/**
* Return whether a fallback match is allowed.
* <p>This is {@code false} by default but may be overridden to return {@code true} in order
* to suggest to an {@link org.springframework.beans.factory.support.AutowireCandidateResolver}
* that a fallback match is acceptable as well.
* @since 4.0
*/
public boolean fallbackMatchAllowed() {
return false;
}
/**
* Return a variant of this descriptor that is intended for a fallback match.
* @since 4.0
* @see #fallbackMatchAllowed()
*/
public DependencyDescriptor forFallbackMatch() {
return new DependencyDescriptor(this) {
@Override
public boolean fallbackMatchAllowed() {
return true;
}
};
}
/**
* Initialize parameter name discovery for the underlying method parameter, if any.
* <p>This method does not actually try to retrieve the parameter name at
* this point; it just allows discovery to happen when the application calls
* {@link #getDependencyName()} (if ever).
*/
public void initParameterNameDiscovery(@Nullable ParameterNameDiscoverer parameterNameDiscoverer) {
if (this.methodParameter != null) {
this.methodParameter.initParameterNameDiscovery(parameterNameDiscoverer);
}
}
/**
* Determine the name of the wrapped parameter/field.
* @return the declared name (never {@code null})
*/
@Nullable
public String getDependencyName() {
return (this.field != null ? this.field.getName() : obtainMethodParameter().getParameterName());
}
/**
* Determine the declared (non-generic) type of the wrapped parameter/field.
* @return the declared type (never {@code null})
*/
public Class<?> getDependencyType() {
if (this.field != null) {
if (this.nestingLevel > 1) {
Type type = this.field.getGenericType();
for (int i = 2; i <= this.nestingLevel; i++) {
if (type instanceof ParameterizedType) {
Type[] args = ((ParameterizedType) type).getActualTypeArguments();
type = args[args.length - 1];
}
}
if (type instanceof Class) {
return (Class<?>) type;
}
else if (type instanceof ParameterizedType) {
Type arg = ((ParameterizedType) type).getRawType();
if (arg instanceof Class) {
return (Class<?>) arg;
}
}
return Object.class;
}
else {
return this.field.getType();
}
}
else {
return obtainMethodParameter().getNestedParameterType();
}
}
@Override
public boolean equals(Object other) {
if (this == other) {
return true;
}
if (!super.equals(other)) {
return false;
}
DependencyDescriptor otherDesc = (DependencyDescriptor) other;
return (this.required == otherDesc.required && this.eager == otherDesc.eager &&
this.nestingLevel == otherDesc.nestingLevel && this.containingClass == otherDesc.containingClass);
}
@Override
public int hashCode() {
return (31 * super.hashCode() + ObjectUtils.nullSafeHashCode(this.containingClass));
}
//---------------------------------------------------------------------
// Serialization support
//---------------------------------------------------------------------
private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
// Rely on default serialization; just initialize state after deserialization.
ois.defaultReadObject();
// Restore reflective handles (which are unfortunately not serializable)
try {
if (this.fieldName != null) {
this.field = this.declaringClass.getDeclaredField(this.fieldName);
}
else {
if (this.methodName != null) {
this.methodParameter = new MethodParameter(
this.declaringClass.getDeclaredMethod(this.methodName, this.parameterTypes), this.parameterIndex);
}
else {
this.methodParameter = new MethodParameter(
this.declaringClass.getDeclaredConstructor(this.parameterTypes), this.parameterIndex);
}
for (int i = 1; i < this.nestingLevel; i++) {
this.methodParameter.increaseNestingLevel();
}
}
}
catch (Throwable ex) {
throw new IllegalStateException("Could not find original class structure", ex);
}
}
/**
* Inner class to avoid a hard dependency on Kotlin at runtime.
*/
private static class KotlinDelegate {
/**
* Check whether the specified {@link Field} represents a nullable Kotlin type or not.
*/
public static boolean isNullable(Field field) {
KProperty<?> property = ReflectJvmMapping.getKotlinProperty(field);
return (property != null && property.getReturnType().isMarkedNullable());
}
}
} 這裡我們來重點看一下主要的屬性：
* private String methodName;
* private Class<?>[] parameterTypes;
* private int parameterIndex;
* private String fieldName;
* private final boolean required;
* private final boolean eager;
* private int nestingLevel = 1;
* private Class<?> containingClass;
* private transient volatile ResolvableType resolvableType;
* private transient volatile TypeDescriptor typeDescriptor; methodName、parameterTypes、parameterIndex、fieldName這幾個屬性分别描述的是方法注入、構造器注入、字段注入相關的資訊。required表示是否是必須的，eager表示的是否為延遲加載，nestingLevel是依賴的層次資訊。containingClass表示的包涵依賴的類資訊。
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