Java面试Spring源码分析 —— Spring IoC
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7、BeanDefinitionValueResolver解析属性值：
当容器在对属性进行依赖注入时，如果发现属性值需要进行类型转换，如属性值是容器中另一个Bean实例对象的引用，则容器首先需要根据属性值解析出所引用的对象，然后才能将该引用对象注入到目标实例对象的属性上去，对属性进行解析的由resolveValueIfNecessary方法实现，其源码如下：
1 //解析属性值，对注入类型进行转换 2 public Object resolveValueIfNecessary(Object argName, Object value) {
3 //对引用类型的属性进行解析 4 if (value instanceof RuntimeBeanReference) {
5 RuntimeBeanReference ref = (RuntimeBeanReference) value;
6 //调用引用类型属性的解析方法 7 return resolveReference(argName, ref);
8 }
9 //对属性值是引用容器中另一个Bean名称的解析 10 else if (value instanceof RuntimeBeanNameReference) {
11 String refName = ((RuntimeBeanNameReference) value).getBeanName();
12 refName = String.valueOf(evaluate(refName));
13 //从容器中获取指定名称的Bean 14 if (!this.beanFactory.containsBean(refName)) {
15 throw new BeanDefinitionStoreException(
16 "Invalid bean name '" + refName + "' in bean reference for " + argName);
17 }
18 return refName;
19 }
20 //对Bean类型属性的解析，主要是Bean中的内部类 21 else if (value instanceof BeanDefinitionHolder) {
22 BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
23 return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
24 }
25 else if (value instanceof BeanDefinition) {
26 BeanDefinition bd = (BeanDefinition) value;
27 return resolveInnerBean(argName, "(inner bean)", bd);
28 }
29 //对集合数组类型的属性解析 30 else if (value instanceof ManagedArray) {
31 ManagedArray array = (ManagedArray) value;
32 //获取数组的类型 33 Class elementType = array.resolvedElementType;
34 if (elementType == null) {
35 //获取数组元素的类型 36 String elementTypeName = array.getElementTypeName();
37 if (StringUtils.hasText(elementTypeName)) {
38 try {
39 //使用反射机制创建指定类型的对象 40 elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
41 array.resolvedElementType = elementType;
42 }
43 catch (Throwable ex) {
44 throw new BeanCreationException(
45 this.beanDefinition.getResourceDescription(), this.beanName,
46 "Error resolving array type for " + argName, ex);
47 }
48 }
49 //没有获取到数组的类型，也没有获取到数组元素的类型，则直接设置数 50 //组的类型为Object 51 else {
52 elementType = Object.class;
53 }
54 }
55 //创建指定类型的数组 56 return resolveManagedArray(argName, (List<?>) value, elementType);
57 }
58 //解析list类型的属性值 59 else if (value instanceof ManagedList) {
60 return resolveManagedList(argName, (List<?>) value);
61 }
62 //解析set类型的属性值 63 else if (value instanceof ManagedSet) {
64 return resolveManagedSet(argName, (Set<?>) value);
65 }
66 //解析map类型的属性值 67 else if (value instanceof ManagedMap) {
68 return resolveManagedMap(argName, (Map<?, ?>) value);
69 }
70 //解析props类型的属性值，props其实就是key和value均为字符串的map 71 else if (value instanceof ManagedProperties) {
72 Properties original = (Properties) value;
73 //创建一个拷贝，用于作为解析后的返回值 74 Properties copy = new Properties();
75 for (Map.Entry propEntry : original.entrySet()) {
76 Object propKey = propEntry.getKey();
77 Object propValue = propEntry.getValue();
78 if (propKey instanceof TypedStringValue) {
79 propKey = evaluate((TypedStringValue) propKey);
80 }
81 if (propValue instanceof TypedStringValue) {
82 propValue = evaluate((TypedStringValue) propValue);
83 }
84 copy.put(propKey, propValue);
85 }
86 return copy;
87 }
88 //解析字符串类型的属性值 89 else if (value instanceof TypedStringValue) {
90 TypedStringValue typedStringValue = (TypedStringValue) value;
91 Object valueObject = evaluate(typedStringValue);
92 try {
93 //获取属性的目标类型 94 Class<?> resolvedTargetType = resolveTargetType(typedStringValue);
95 if (resolvedTargetType != null) {
96 //对目标类型的属性进行解析，递归调用 97 return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
98 }
99 //没有获取到属性的目标对象，则按Object类型返回 100 else {
101 return valueObject;
102 }
103 }
104 catch (Throwable ex) {
105 throw new BeanCreationException(
106 this.beanDefinition.getResourceDescription(), this.beanName,
107 "Error converting typed String value for " + argName, ex);
108 }
109 }
110 else {
111 return evaluate(value);
112 }
113 }
114 //解析引用类型的属性值 115 private Object resolveReference(Object argName, RuntimeBeanReference ref) {
116 try {
117 //获取引用的Bean名称 118 String refName = ref.getBeanName();
119 refName = String.valueOf(evaluate(refName));
120 //如果引用的对象在父类容器中，则从父类容器中获取指定的引用对象 121 if (ref.isToParent()) {
122 if (this.beanFactory.getParentBeanFactory() == null) {
123 throw new BeanCreationException(
124 this.beanDefinition.getResourceDescription(), this.beanName,
125 "Can't resolve reference to bean '" + refName +
126 "' in parent factory: no parent factory available");
127 }
128 return this.beanFactory.getParentBeanFactory().getBean(refName);
129 }
130 //从当前的容器中获取指定的引用Bean对象，如果指定的Bean没有被实例化 131 //则会递归触发引用Bean的初始化和依赖注入 132 else {
133 Object bean = this.beanFactory.getBean(refName);
134 //将当前实例化对象的依赖引用对象 135 this.beanFactory.registerDependentBean(refName, this.beanName);
136 return bean;
137 }
138 }
139 catch (BeansException ex) {
140 throw new BeanCreationException(
141 this.beanDefinition.getResourceDescription(), this.beanName,
142 "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
143 }
144 }
145 //解析array类型的属性 146 private Object resolveManagedArray(Object argName, List<?> ml, Class elementType) {
147 //创建一个指定类型的数组，用于存放和返回解析后的数组 148 Object resolved = Array.newInstance(elementType, ml.size());
149 for (int i = 0; i < ml.size(); i++) {
150 //递归解析array的每一个元素，并将解析后的值设置到resolved数组中，索引为i 151 Array.set(resolved, i,
152 resolveValueIfNecessary(new KeyedArgName(argName, i), ml.get(i)));
153 }
154 return resolved;
155 }
156 //解析list类型的属性 157 private List resolveManagedList(Object argName, List<?> ml) {
158 List<Object> resolved = new ArrayList<Object>(ml.size());
159 for (int i = 0; i < ml.size(); i++) {
160 //递归解析list的每一个元素 161 resolved.add(
162 resolveValueIfNecessary(new KeyedArgName(argName, i), ml.get(i)));
163 }
164 return resolved;
165 }
166 //解析set类型的属性 167 private Set resolveManagedSet(Object argName, Set<?> ms) {
168 Set<Object> resolved = new LinkedHashSet<Object>(ms.size());
169 int i = 0;
170 //递归解析set的每一个元素 171 for (Object m : ms) {
172 resolved.add(resolveValueIfNecessary(new KeyedArgName(argName, i), m));
173 i++;
174 }
175 return resolved;
176 }
177 //解析map类型的属性 178 private Map resolveManagedMap(Object argName, Map<?, ?> mm) {
179 Map<Object, Object> resolved = new LinkedHashMap<Object, Object>(mm.size());
180 //递归解析map中每一个元素的key和value 181 for (Map.Entry entry : mm.entrySet()) {
182 Object resolvedKey = resolveValueIfNecessary(argName, entry.getKey());
183 Object resolvedValue = resolveValueIfNecessary(
184 new KeyedArgName(argName, entry.getKey()), entry.getValue());
185 resolved.put(resolvedKey, resolvedValue);
186 }
187 return resolved;
188 }
通过上面的代码分析，我们明白了Spring是如何将引用类型，内部类以及集合类型等属性进行解析的，属性值解析完成后就可以进行依赖注入了，依赖注入的过程就是Bean对象实例设置到它所依赖的Bean对象属性上去，在第7步中我们已经说过，依赖注入是通过bw.setPropertyValues方法实现的，该方法也使用了委托模式，在BeanWrapper接口中至少定义了方法声明，依赖注入的具体实现交由其实现类BeanWrapperImpl来完成，下面我们就分析依BeanWrapperImpl中赖注入相关的源码。
8、BeanWrapperImpl对Bean属性的依赖注入：
BeanWrapperImpl类主要是对容器中完成初始化的Bean实例对象进行属性的依赖注入，即把Bean对象设置到它所依赖的另一个Bean的属性中去，依赖注入的相关源码如下：
1 //实现属性依赖注入功能 2 private void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
3 //PropertyTokenHolder主要保存属性的名称、路径，以及集合的size等信息 4 String propertyName = tokens.canonicalName;
5 String actualName = tokens.actualName;
6 //keys是用来保存集合类型属性的size 7 if (tokens.keys != null) {
8 //将属性信息拷贝 9 PropertyTokenHolder getterTokens = new PropertyTokenHolder();
10 getterTokens.canonicalName = tokens.canonicalName;
11 getterTokens.actualName = tokens.actualName;
12 getterTokens.keys = new String[tokens.keys.length - 1];
13 System.arraycopy(tokens.keys, 0, getterTokens.keys, 0, tokens.keys.length - 1);
14 Object propValue;
15 try {
16 //获取属性值，该方法内部使用JDK的内省( Introspector)机制，调用属性//的getter(readerMethod)方法，获取属性的值 17 propValue = getPropertyValue(getterTokens);
18 }
19 catch (NotReadablePropertyException ex) {
20 throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
21 "Cannot access indexed value in property referenced " +
22 "in indexed property path '" + propertyName + "'", ex);
23 }
24 //获取集合类型属性的长度 25 String key = tokens.keys[tokens.keys.length - 1];
26 if (propValue == null) {
27 throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
28 "Cannot access indexed value in property referenced " +
29 "in indexed property path '" + propertyName + "': returned null");
30 }
31 //注入array类型的属性值 32 else if (propValue.getClass().isArray()) {
33 //获取属性的描述符 34 PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
35 //获取数组的类型 36 Class requiredType = propValue.getClass().getComponentType();
37 //获取数组的长度 38 int arrayIndex = Integer.parseInt(key);
39 Object oldValue = null;
40 try {
41 //获取数组以前初始化的值 42 if (isExtractOldValueForEditor()) {
43 oldValue = Array.get(propValue, arrayIndex);
44 }
45 //将属性的值赋值给数组中的元素 46 Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(), requiredType,
47 new PropertyTypeDescriptor(pd, new MethodParameter(pd.getReadMethod(), -1), requiredType));
48 Array.set(propValue, arrayIndex, convertedValue);
49 }
50 catch (IndexOutOfBoundsException ex) {
51 throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
52 "Invalid array index in property path '" + propertyName + "'", ex);
53 }
54 }
55 //注入list类型的属性值 56 else if (propValue instanceof List) {
57 PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
58 //获取list集合的类型 59 Class requiredType = GenericCollectionTypeResolver.getCollectionReturnType(
60 pd.getReadMethod(), tokens.keys.length);
61 List list = (List) propValue;
62 //获取list集合的size 63 int index = Integer.parseInt(key);
64 Object oldValue = null;
65 if (isExtractOldValueForEditor() && index < list.size()) {
66 oldValue = list.get(index);
67 }
68 //获取list解析后的属性值 69 Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(), requiredType,
70 new PropertyTypeDescriptor(pd, new MethodParameter(pd.getReadMethod(), -1), requiredType));
71 if (index < list.size()) {
72 //为list属性赋值 73 list.set(index, convertedValue);
74 }
75 //如果list的长度大于属性值的长度，则多余的元素赋值为null 76 else if (index >= list.size()) {
77 for (int i = list.size(); i < index; i++) {
78 try {
79 list.add(null);
80 }
81 catch (NullPointerException ex) {
82 throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
83 "Cannot set element with index " + index + " in List of size " +
84 list.size() + ", accessed using property path '" + propertyName +
85 "': List does not support filling up gaps with null elements");
86 }
87 }
88 list.add(convertedValue);
89 }
90 }
91 //注入map类型的属性值 92 else if (propValue instanceof Map) {
93 PropertyDescriptor pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
94 //获取map集合key的类型 95 Class mapKeyType = GenericCollectionTypeResolver.getMapKeyReturnType(
96 pd.getReadMethod(), tokens.keys.length);
97 //获取map集合value的类型 98 Class mapValueType = GenericCollectionTypeResolver.getMapValueReturnType(
99 pd.getReadMethod(), tokens.keys.length);
100 Map map = (Map) propValue;
101 //解析map类型属性key值 102 Object convertedMapKey = convertIfNecessary(null, null, key, mapKeyType,
103 new PropertyTypeDescriptor(pd, new MethodParameter(pd.getReadMethod(), -1), mapKeyType));
104 Object oldValue = null;
105 if (isExtractOldValueForEditor()) {
106 oldValue = map.get(convertedMapKey);
107 }
108 //解析map类型属性value值 109 Object convertedMapValue = convertIfNecessary(
110 propertyName, oldValue, pv.getValue(), mapValueType,
111 new TypeDescriptor(new MethodParameter(pd.getReadMethod(), -1, tokens.keys.length + 1)));
112 //将解析后的key和value值赋值给map集合属性 113 map.put(convertedMapKey, convertedMapValue);
114 }
115 else {
116 throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
117 "Property referenced in indexed property path '" + propertyName +
118 "' is neither an array nor a List nor a Map; returned value was [" + pv.getValue() + "]");
119 }
120 }
121 //对非集合类型的属性注入 122 else {
123 PropertyDescriptor pd = pv.resolvedDescriptor;
124 if (pd == null || !pd.getWriteMethod().getDeclaringClass().isInstance(this.object)) {
125 pd = getCachedIntrospectionResults().getPropertyDescriptor(actualName);
126 //无法获取到属性名或者属性没有提供setter(写方法)方法 127 if (pd == null || pd.getWriteMethod() == null) {
128 //如果属性值是可选的，即不是必须的，则忽略该属性值 129 if (pv.isOptional()) {
130 logger.debug("Ignoring optional value for property '" + actualName +
131 "' - property not found on bean class [" + getRootClass().getName() + "]");
132 return;
133 }
134 //如果属性值是必须的，则抛出无法给属性赋值，因为每天提供setter方法异常 135 else {
136 PropertyMatches matches = PropertyMatches.forProperty(propertyName, getRootClass());
137 throw new NotWritablePropertyException(
138 getRootClass(), this.nestedPath + propertyName,
139 matches.buildErrorMessage(), matches.getPossibleMatches());
140 }
141 }
142 pv.getOriginalPropertyValue().resolvedDescriptor = pd;
143 }
144 Object oldValue = null;
145 try {
146 Object originalValue = pv.getValue();
147 Object valueToApply = originalValue;
148 if (!Boolean.FALSE.equals(pv.conversionNecessary)) {
149 if (pv.isConverted()) {
150 valueToApply = pv.getConvertedValue();
151 }
152 else {
153 if (isExtractOldValueForEditor() && pd.getReadMethod() != null) {
154 //获取属性的getter方法(读方法)，JDK内省机制 155 final Method readMethod = pd.getReadMethod();
156 //如果属性的getter方法不是public访问控制权限的，即访问控制权限比较严格， 157 //则使用JDK的反射机制强行访问非public的方法(暴力读取属性值) 158 if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers()) &&
159 !readMethod.isAccessible()) {
160 if (System.getSecurityManager()!= null) {
161 //匿名内部类，根据权限修改属性的读取控制限制 162 AccessController.doPrivileged(new PrivilegedAction<Object>() {
163 public Object run() {
164 readMethod.setAccessible(true);
165 return null;
166 }
167 });
168 }
169 else {
170 readMethod.setAccessible(true);
171 }
172 }
173 try {
174 //属性没有提供getter方法时，调用潜在的读取属性值//的方法，获取属性值 175 if (System.getSecurityManager() != null) {
176 oldValue = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
177 public Object run() throws Exception {
178 return readMethod.invoke(object);
179 }
180 }, acc);
181 }
182 else {
183 oldValue = readMethod.invoke(object);
184 }
185 }
186 catch (Exception ex) {
187 if (ex instanceof PrivilegedActionException) {
188 ex = ((PrivilegedActionException) ex).getException();
189 }
190 if (logger.isDebugEnabled()) {
191 logger.debug("Could not read previous value of property '" +
192 this.nestedPath + propertyName + "'", ex);
193 }
194 }
195 }
196 //设置属性的注入值 197 valueToApply = convertForProperty(propertyName, oldValue, originalValue, pd);
198 }
199 pv.getOriginalPropertyValue().conversionNecessary = (valueToApply != originalValue);
200 }
201 //根据JDK的内省机制，获取属性的setter(写方法)方法 202 final Method writeMethod = (pd instanceof GenericTypeAwarePropertyDescriptor ?
203 ((GenericTypeAwarePropertyDescriptor) pd).getWriteMethodForActualAccess() :
204 pd.getWriteMethod());
205 //如果属性的setter方法是非public，即访问控制权限比较严格，则使用JDK的反射机制， 206 //强行设置setter方法可访问(暴力为属性赋值) 207 if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers()) && !writeMethod.isAccessible()) {
208 //如果使用了JDK的安全机制，则需要权限验证 209 if (System.getSecurityManager()!= null) {
210 AccessController.doPrivileged(new PrivilegedAction<Object>() {
211 public Object run() {
212 writeMethod.setAccessible(true);
213 return null;
214 }
215 });
216 }
217 else {
218 writeMethod.setAccessible(true);
219 }
220 }
221 final Object value = valueToApply;
222 if (System.getSecurityManager() != null) {
223 try {
224 //将属性值设置到属性上去 225 AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
226 public Object run() throws Exception {
227 writeMethod.invoke(object, value);
228 return null;
229 }
230 }, acc);
231 }
232 catch (PrivilegedActionException ex) {
233 throw ex.getException();
234 }
235 }
236 else {
237 writeMethod.invoke(this.object, value);
238 }
239 }
240 catch (TypeMismatchException ex) {
241 throw ex;
242 }
243 catch (InvocationTargetException ex) {
244 PropertyChangeEvent propertyChangeEvent =
245 new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
246 if (ex.getTargetException() instanceof ClassCastException) {
247 throw new TypeMismatchException(propertyChangeEvent, pd.getPropertyType(), ex.getTargetException());
248 }
249 else {
250 throw new MethodInvocationException(propertyChangeEvent, ex.getTargetException());
251 }
252 }
253 catch (Exception ex) {
254 PropertyChangeEvent pce =
255 new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
256 throw new MethodInvocationException(pce, ex);
257 }
258 }
}
通过对上面注入依赖代码的分析，我们已经明白了Spring IoC容器是如何将属性的值注入到Bean实例对象中去的：
(1).对于集合类型的属性，将其属性值解析为目标类型的集合后直接赋值给属性。
(2).对于非集合类型的属性，大量使用了JDK的反射和内省机制，通过属性的getter方法(reader method)获取指定属性注入以前的值，同时调用属性的setter方法(writer method)为属性设置注入后的值。看到这里相信很多人都明白了Spring的setter注入原理。
至此Spring IoC容器对Bean定义资源文件的定位，载入、解析和依赖注入已经全部分析完毕，现在Spring IoC容器中管理了一系列靠依赖关系联系起来的Bean，程序不需要应用自己手动创建所需的对象，Spring IoC容器会在我们使用的时候自动为我们创建，并且为我们注入好相关的依赖，这就是Spring核心功能的控制反转和依赖注入的相关功能。
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