基於Mybatis-3.5.0版本
1.0 Reflector反射器
org.apache.ibatis.reflection.Reflector反射器，每個Reflector對應一個類，會快取反射操作需要的類的後設資料，例如：構造方法、屬性名、get/set方法等等。程式碼如下：
/**
* This class represents a cached set of class definition information that
* allows for easy mapping between property names and getter/setter methods.
*
* 反射器，每個 Reflector對應一個類， 會快取類的元資訊，
* 此類表示一組快取的類的後設資料，允許在屬性名和getter/setter方法之間輕鬆對映。
* @author Clinton Begin
*/
public class Reflector {
// 對應類的Class物件
private final Class<?> type;
// 可讀屬性陣列
private final String[] readablePropertyNames;
// 可寫屬性陣列
private final String[] writeablePropertyNames;
// key是屬性名稱，value是Invoker物件
/**
* Invoker介面：介面卡模式用於消除java bean的
* getter方法、setter方法、Filed屬性的set/get的操作差異
*/
private final Map<String, Invoker> setMethods = new HashMap<>();
// key是屬性名稱，value是Invoker物件
private final Map<String, Invoker> getMethods = new HashMap<>();
// key是屬性名稱，value是setter方法的引數值型別
private final Map<String, Class<?>> setTypes = new HashMap<>();
// key是屬性名稱，value是getter方法的返回值型別
private final Map<String, Class<?>> getTypes = new HashMap<>();
// 預設無參構造器
private Constructor<?> defaultConstructor;
// 不區分大小寫的屬性集合 key:toUpperCase的屬性名稱 value：原屬性名稱
private Map<String, String> caseInsensitivePropertyMap = new HashMap<>();
public Reflector(Class<?> clazz) {
type = clazz;
addDefaultConstructor(clazz);
addGetMethods(clazz);
addSetMethods(clazz);
addFields(clazz);
readablePropertyNames = getMethods.keySet().toArray(new String[getMethods.keySet().size()]);
writeablePropertyNames = setMethods.keySet().toArray(new String[setMethods.keySet().size()]);
for (String propName : readablePropertyNames) {
caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
}
for (String propName : writeablePropertyNames) {
caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
}
}
/**
* 初始化defaultConstructor
* @param clazz
*/
private void addDefaultConstructor(Class<?> clazz) {
// 獲取所有宣告的構造方法
Constructor<?>[] consts = clazz.getDeclaredConstructors();
for (Constructor<?> constructor : consts) {
// 引數數量為0，即沒有引數的構造方法為 預設構造方法
if (constructor.getParameterTypes().length == 0) {
this.defaultConstructor = constructor;
}
}
}
/**
* 初始化getMethods和getTypes
* @param cls
*/
private void addGetMethods(Class<?> cls) {
// key：屬性名稱，value：getter方法集合
/**
* 因為在java的繼承關係中，會存在子類重寫父類的方法，但是放大了返回值型別 所有會存在conflicting衝突的方法
* 例：
* super： List<String> getIds();
* sub: ArrayList<String> getIds();
*/
Map<String, List<Method>> conflictingGetters = new HashMap<>();
// 獲取所有方法
Method[] methods = getClassMethods(cls);
/**
* 迴圈新增規範的getter方法
* java bean getter規範：
* 1.沒有引數
* 2.若以get開頭，方法長度大於3
* 3.若以is開頭，方法長度大於2
*/
for (Method method : methods) {
if (method.getParameterTypes().length > 0) {
continue;
}
String name = method.getName();
if ((name.startsWith("get") && name.length() > 3) || (name.startsWith("is") && name.length() > 2)) {
name = PropertyNamer.methodToProperty(name);
addMethodConflict(conflictingGetters, name, method);
}
}
// 解決衝突的方法，將最合適的方法新增到getMethods和getTypes
resolveGetterConflicts(conflictingGetters);
}
/**
* 解決getter方法衝突，尋找最規範和最合理的getter方法
* 衝突原因：因為在java的繼承關係中，會存在子類重寫父類的方法，但是放大了返回值型別 所有會存在conflicting衝突的方法
* 例：
* super： List<String> getIds();
* sub: ArrayList<String> getIds();
*
* @param conflictingGetters
*/
private void resolveGetterConflicts(Map<String, List<Method>> conflictingGetters) {
for (Entry<String, List<Method>> entry : conflictingGetters.entrySet()) {
Method winner = null;//勝利者 即最規範最合理的getter方法
String propName = entry.getKey();
for (Method candidate : entry.getValue()) {
if (winner == null) {
winner = candidate;
continue;
}
/**
* 如果返回型別相同,不是boolean型別報異常,以is開頭的方法為主
* 主要解決Boolean型別屬性，命名不規範 例： getFlag() 與 isFlag()
*/
Class<?> winnerType = winner.getReturnType();
Class<?> candidateType = candidate.getReturnType();
if (candidateType.equals(winnerType)) {
if (!boolean.class.equals(candidateType)) {
throw new ReflectionException(
"Illegal overloaded getter method with ambiguous type for property " + propName
+ " in class " + winner.getDeclaringClass()
+ ". This breaks the JavaBeans specification and can cause unpredictable results.");
} else if (candidate.getName().startsWith("is")) {
winner = candidate;
}
/**
* isAssignableFrom()方法是從類繼承的角度去判斷，判斷是否為某個類的父類
* instanceof()方法是從例項繼承的角度去判斷,是判斷是否某個類的子類。
* 以子類返回型別為主：針對一些重寫的場景，子類放大了返回值
* 例如: 父類的一個方法的返回值為 List ，子類對該方法的返回值可以覆寫為 ArrayList
*/
} else if (candidateType.isAssignableFrom(winnerType)) {
// OK getter type is descendant
} else if (winnerType.isAssignableFrom(candidateType)) {
winner = candidate;
} else {
throw new ReflectionException("Illegal overloaded getter method with ambiguous type for property "
+ propName + " in class " + winner.getDeclaringClass()
+ ". This breaks the JavaBeans specification and can cause unpredictable results.");
}
}
addGetMethod(propName, winner);
}
}
/**
* 快取Getter方法和Getter方法的返回值型別
* @param name
* @param method
*/
private void addGetMethod(String name, Method method) {
if (isValidPropertyName(name)) {
getMethods.put(name, new MethodInvoker(method));
Type returnType = TypeParameterResolver.resolveReturnType(method, type);
getTypes.put(name, typeToClass(returnType));
}
}
/**
* 初始化 setMethods和setTypes屬性
* 整個流程和初始化Getter方法類似
* @param cls
*/
private void addSetMethods(Class<?> cls) {
Map<String, List<Method>> conflictingSetters = new HashMap<>();
Method[] methods = getClassMethods(cls);
for (Method method : methods) {
String name = method.getName();
if (name.startsWith("set") && name.length() > 3) {
if (method.getParameterTypes().length == 1) {
name = PropertyNamer.methodToProperty(name);
addMethodConflict(conflictingSetters, name, method);
}
}
}
resolveSetterConflicts(conflictingSetters);
}
/**
* java8 Map default computeIfAbsent方法
* put key，存在則返回value，不存在建立value並put(key,vlue) 返回value
* @param conflictingMethods
* @param name
* @param method
*/
private void addMethodConflict(Map<String, List<Method>> conflictingMethods, String name, Method method) {
List<Method> list = conflictingMethods.computeIfAbsent(name, k -> new ArrayList<>());
list.add(method);
}
/**
* 解決Setter方法衝突，保證最規範在最合理的Setter方法
* @param conflictingSetters
*/
private void resolveSetterConflicts(Map<String, List<Method>> conflictingSetters) {
for (String propName : conflictingSetters.keySet()) {
List<Method> setters = conflictingSetters.get(propName);
Class<?> getterType = getTypes.get(propName);
Method match = null;
ReflectionException exception = null;
for (Method setter : setters) {
Class<?> paramType = setter.getParameterTypes()[0];
//setter對應的引數型別與該屬性對應的getter的響應值型別一致則是合理的setter方法
if (paramType.equals(getterType)) {
// should be the best match
match = setter;
break;
}
if (exception == null) {
try {
match = pickBetterSetter(match, setter, propName);
} catch (ReflectionException e) {
// there could still be the 'best match'
match = null;
exception = e;
}
}
}
if (match == null) {
throw exception;
} else {
addSetMethod(propName, match);
}
}
}
private Method pickBetterSetter(Method setter1, Method setter2, String property) {
if (setter1 == null) {
return setter2;
}
Class<?> paramType1 = setter1.getParameterTypes()[0];
Class<?> paramType2 = setter2.getParameterTypes()[0];
if (paramType1.isAssignableFrom(paramType2)) {
return setter2;
} else if (paramType2.isAssignableFrom(paramType1)) {
return setter1;
}
throw new ReflectionException(
"Ambiguous setters defined for property '" + property + "' in class '" + setter2.getDeclaringClass()
+ "' with types '" + paramType1.getName() + "' and '" + paramType2.getName() + "'.");
}
private void addSetMethod(String name, Method method) {
if (isValidPropertyName(name)) {
setMethods.put(name, new MethodInvoker(method));
Type[] paramTypes = TypeParameterResolver.resolveParamTypes(method, type);
setTypes.put(name, typeToClass(paramTypes[0]));
}
}
private Class<?> typeToClass(Type src) {
Class<?> result = null;
if (src instanceof Class) {
result = (Class<?>) src;
} else if (src instanceof ParameterizedType) {
result = (Class<?>) ((ParameterizedType) src).getRawType();
} else if (src instanceof GenericArrayType) {
Type componentType = ((GenericArrayType) src).getGenericComponentType();
if (componentType instanceof Class) {
result = Array.newInstance((Class<?>) componentType, 0).getClass();
} else {
Class<?> componentClass = typeToClass(componentType);
result = Array.newInstance(componentClass, 0).getClass();
}
}
if (result == null) {
result = Object.class;
}
return result;
}
/**
* 初始化setMethods和setTypes屬性和getMethods和getTypes屬性
* 主要處理一些屬性沒有對應的getter和setter方法的情況
* @param clazz
*/
private void addFields(Class<?> clazz) {
// 獲取宣告的屬性物件
Field[] fields = clazz.getDeclaredFields();
for (Field field : fields) {
// setter快取裡面不存在的就處理
if (!setMethods.containsKey(field.getName())) {
// issue #379 - removed the check for final because JDK 1.5 allows
// modification of final fields through reflection (JSR-133). (JGB)
// pr #16 - final static can only be set by the classloader
// 過濾掉final和static修飾的欄位
int modifiers = field.getModifiers();
if (!(Modifier.isFinal(modifiers) && Modifier.isStatic(modifiers))) {
addSetField(field);
}
}
// getter快取裡面不存在的就處理
if (!getMethods.containsKey(field.getName())) {
addGetField(field);
}
}
// 如果有父類則遞迴處理
if (clazz.getSuperclass() != null) {
addFields(clazz.getSuperclass());
}
}
private void addSetField(Field field) {
if (isValidPropertyName(field.getName())) {
setMethods.put(field.getName(), new SetFieldInvoker(field));
Type fieldType = TypeParameterResolver.resolveFieldType(field, type);
setTypes.put(field.getName(), typeToClass(fieldType));
}
}
private void addGetField(Field field) {
if (isValidPropertyName(field.getName())) {
getMethods.put(field.getName(), new GetFieldInvoker(field));
Type fieldType = TypeParameterResolver.resolveFieldType(field, type);
getTypes.put(field.getName(), typeToClass(fieldType));
}
}
private boolean isValidPropertyName(String name) {
return !(name.startsWith("$") || "serialVersionUID".equals(name) || "class".equals(name));
}
/**
* This method returns an array containing all methods declared in this class
* and any superclass. We use this method, instead of the simpler
* Class.getMethods(), because we want to look for private methods as well.
*
* @param cls The class
* @return An array containing all methods in this class
*/
private Method[] getClassMethods(Class<?> cls) {
// key:方法簽名，value：Method方法
Map<String, Method> uniqueMethods = new HashMap<>();
Class<?> currentClass = cls;
// 迴圈類，類的父類，類的父類的父類，直到父類為 Object
while (currentClass != null && currentClass != Object.class) {
// 記錄當前類定義的方法
addUniqueMethods(uniqueMethods, currentClass.getDeclaredMethods());
// we also need to look for interface methods -
// because the class may be abstract
// 記錄介面中定義的方法,因為這個類可能是抽象類
Class<?>[] interfaces = currentClass.getInterfaces();
for (Class<?> anInterface : interfaces) {
// 記錄介面定義的方法
addUniqueMethods(uniqueMethods, anInterface.getMethods());
}
// 獲得父類
currentClass = currentClass.getSuperclass();
}
Collection<Method> methods = uniqueMethods.values();
return methods.toArray(new Method[methods.size()]);
}
/**
* 新增方法簽名唯一的方法
* @param uniqueMethods
* @param methods
*/
private void addUniqueMethods(Map<String, Method> uniqueMethods, Method[] methods) {
for (Method currentMethod : methods) {
if (!currentMethod.isBridge()) {// bridge方法不處理
// 獲取方法簽名
String signature = getSignature(currentMethod);
// check to see if the method is already known
// if it is known, then an extended class must have
// overridden a method
if (!uniqueMethods.containsKey(signature)) {
uniqueMethods.put(signature, currentMethod);
}
}
}
}
/**
* 獲取方法簽名
* 格式：響應值型別名稱方法名稱:方法引數型別名稱
* 例：
* void#setId:java.lang.Long
* @param method
* @return
*/
private String getSignature(Method method) {
StringBuilder sb = new StringBuilder();
Class<?> returnType = method.getReturnType();
if (returnType != null) {
sb.append(returnType.getName()).append('#');
}
sb.append(method.getName());
Class<?>[] parameters = method.getParameterTypes();
for (int i = 0; i < parameters.length; i++) {
if (i == 0) {
sb.append(':');
} else {
sb.append(',');
}
sb.append(parameters[i].getName());
}
return sb.toString();
}
/**
* Checks whether can control member accessible.
* 檢查是否可以控制成員的訪問性
* @return If can control member accessible, it return {@literal true}
* @since 3.5.0
*/
public static boolean canControlMemberAccessible() {
try {
SecurityManager securityManager = System.getSecurityManager();
if (null != securityManager) {
securityManager.checkPermission(new ReflectPermission("suppressAccessChecks"));
}
} catch (SecurityException e) {
return false;
}
return true;
}
/**
* Gets the name of the class the instance provides information for
*
* @return The class name
*/
public Class<?> getType() {
return type;
}
public Constructor<?> getDefaultConstructor() {
if (defaultConstructor != null) {
return defaultConstructor;
} else {
throw new ReflectionException("There is no default constructor for " + type);
}
}
public boolean hasDefaultConstructor() {
return defaultConstructor != null;
}
public Invoker getSetInvoker(String propertyName) {
Invoker method = setMethods.get(propertyName);
if (method == null) {
throw new ReflectionException(
"There is no setter for property named '" + propertyName + "' in '" + type + "'");
}
return method;
}
public Invoker getGetInvoker(String propertyName) {
Invoker method = getMethods.get(propertyName);
if (method == null) {
throw new ReflectionException(
"There is no getter for property named '" + propertyName + "' in '" + type + "'");
}
return method;
}
/**
* Gets the type for a property setter
*
* @param propertyName - the name of the property
* @return The Class of the property setter
*/
public Class<?> getSetterType(String propertyName) {
Class<?> clazz = setTypes.get(propertyName);
if (clazz == null) {
throw new ReflectionException(
"There is no setter for property named '" + propertyName + "' in '" + type + "'");
}
return clazz;
}
/**
* Gets the type for a property getter
*
* @param propertyName - the name of the property
* @return The Class of the property getter
*/
public Class<?> getGetterType(String propertyName) {
Class<?> clazz = getTypes.get(propertyName);
if (clazz == null) {
throw new ReflectionException(
"There is no getter for property named '" + propertyName + "' in '" + type + "'");
}
return clazz;
}
/**
* Gets an array of the readable properties for an object
*
* @return The array
*/
public String[] getGetablePropertyNames() {
return readablePropertyNames;
}
/**
* Gets an array of the writable properties for an object
*
* @return The array
*/
public String[] getSetablePropertyNames() {
return writeablePropertyNames;
}
/**
* Check to see if a class has a writable property by name
*
* @param propertyName - the name of the property to check
* @return True if the object has a writable property by the name
*/
public boolean hasSetter(String propertyName) {
return setMethods.keySet().contains(propertyName);
}
/**
* Check to see if a class has a readable property by name
*
* @param propertyName - the name of the property to check
* @return True if the object has a readable property by the name
*/
public boolean hasGetter(String propertyName) {
return getMethods.keySet().contains(propertyName);
}
public String findPropertyName(String name) {
return caseInsensitivePropertyMap.get(name.toUpperCase(Locale.ENGLISH));
}
}
複製程式碼2.0 ReflectorFactory
org.apache.ibatis.reflection.ReflectorFactoryReflector工廠介面用於直接根據Class物件建立Reflector物件，並提供了快取功能。程式碼如下：
public interface ReflectorFactory {
boolean isClassCacheEnabled();
void setClassCacheEnabled(boolean classCacheEnabled);
Reflector findForClass(Class<?> type);
}
複製程式碼2.1 DefaultReflectorFactory
org.apache.ibatis.reflection.DefaultReflectorFactory預設Reflector工廠介面實現類，程式碼如下：
public class DefaultReflectorFactory implements ReflectorFactory {
// 是否開啟快取
private boolean classCacheEnabled = true;
/**
* ConcurrentMap 快取Reflector資訊 key：Class value:Class對應的Reflector
*/
private final ConcurrentMap<Class<?>, Reflector> reflectorMap = new ConcurrentHashMap<>();
public DefaultReflectorFactory() {
}
@Override
public boolean isClassCacheEnabled() {
return classCacheEnabled;
}
@Override
public void setClassCacheEnabled(boolean classCacheEnabled) {
this.classCacheEnabled = classCacheEnabled;
}
/**
* 開啟快取：則直接從reflectorMap獲取，未獲取到則建立Reflector物件放入map並返回
* 未開啟快取：每次都建立新的Reflector物件
*/
@Override
public Reflector findForClass(Class<?> type) {
if (classCacheEnabled) {
// synchronized (type) removed see issue #461
return reflectorMap.computeIfAbsent(type, Reflector::new);
} else {
return new Reflector(type);
}
}
}
複製程式碼3.0 整理學習的知識點
3.1 java8 Map computeIfAbsent
java8 新特性：default方法
預設方法使得開發者可以在不破壞二進位制相容性的前提下，往現存介面中新增新的方法，即不強制那些實現了該介面的類也同時實現這個新加的方法。
computeIfAbsent方法為Map類下面的預設方法，程式碼如下：
//put key，存在則返回value，不存在建立value並put(key,vlue) 返回value
default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
Objects.requireNonNull(mappingFunction);
V v;
if ((v = get(key)) == null) {
V newValue;
if ((newValue = mappingFunction.apply(key)) != null) {
put(key, newValue);
return newValue;
}
}
return v;
}
複製程式碼特別適用於一些value為集合型別的資料，可以看下Mybatis3.4.6版本和3.5.0版本的程式碼差異:
//3.5.0
private void addMethodConflict(Map<String, List<Method>> conflictingMethods, String name, Method method) {
List<Method> list = conflictingMethods.computeIfAbsent(name, k -> new ArrayList<>());
list.add(method);
}
//3.4.6
private void addMethodConflict(Map<String, List<Method>> conflictingMethods, String name, Method method) {
List<Method> list = conflictingMethods.get(name);
if (list == null) {
list = new ArrayList<Method>();
conflictingMethods.put(name, list);
}
list.add(method);
}
複製程式碼3.2 關於Method isBridge()
關於Method isBridge()方法，主要針對一些泛型方法，因為泛型擦除的原因，導致過載的方法失去意義，所以會在對應的類裡面注入一個bridge方法，見下例： Java反射中method.isBridge()由來,含義和使用場景
interface A<T> {
void func(T t);
}
class B implements A<String> {
@Override
public void func(String t) {
System.out.println(t);
}
}
public static void main(String[] args) throws Exception {
B obj = new B();
Method func = B.class.getMethod("func", String.class);
func.invoke(obj, "AAA");
System.out.println(func.isBridge());
func = B.class.getMethod("func", Object.class);
func.invoke(obj, "BBB");
System.out.println(func.isBridge());
}
console：
AAA
false
BBB
true
複製程式碼3.3 Class isAssignableFrom方法
isAssignableFrom 與 instanceof的差異：
- isAssignableFrom 法是從類繼承的角度去判斷，判斷是否為某個類的父類
- instanceof 是從例項繼承的角度去判斷,是判斷是否某個類的子類
例：
List<String> list = new ArrayList<>();
ArrayList.class.isAssignableFrom(List.class)// false
list instanceof List // true
複製程式碼4.0 總結
看完Reflector的程式碼，小菜阿甘有3個方面的感悟：
- 1 大佬對java基礎認識很深刻，考慮問題非常全面
- 2 方法命名和變數命名非常規範和直白（是這個原因導致註釋這麼少嗎o(╥﹏╥)o）
- 3 大家平時在開發的過程中一定要有很好的規範，例如：如果大家javabean的getter或setter命名不規範，用Mybatis一定會有一些莫名其妙的問題
失控的阿甘，樂於分享，記錄點滴