下面是一个简单的Mapper接口调用，首先同个session的getMapper方法获取Mapper的代理对象，然后通过代理对象去调用Mapper接口的方法
EmployeeMapper mapper = session.getMapper(EmployeeMapper.class);
Employee employee = mapper.getEmpByIdAndName(1,"tom");
Employee getEmpByIdAndName(@Param("id") Integer id, @Param("name") String name, int age);
源码分析：
首先看MapperProxy类，关键是mapperMethod.execute(this.sqlSession, args);
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
}
if (method.isDefault()) {
return this.invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable var5) {
throw ExceptionUtil.unwrapThrowable(var5);
}
MapperMethod mapperMethod = this.cachedMapperMethod(method);
return mapperMethod.execute(this.sqlSession, args);
}
MapperMethod类：execute方法中，在调用session操作之前，都会调用this.method.convertArgsToSqlCommandParam(args),最终还是调用ParamNameResolver类
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
Object param;
switch(this.command.getType()) {
case INSERT:
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
break;
case UPDATE:
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
break;
case DELETE:
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
break;
case SELECT:
if (this.method.returnsVoid() && this.method.hasResultHandler()) {
this.executeWithResultHandler(sqlSession, args);
result = null;
} else if (this.method.returnsMany()) {
result = this.executeForMany(sqlSession, args);
} else if (this.method.returnsMap()) {
result = this.executeForMap(sqlSession, args);
} else if (this.method.returnsCursor()) {
result = this.executeForCursor(sqlSession, args);
} else {
param = this.method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(this.command.getName(), param);
if (this.method.returnsOptional() && (result == null || !this.method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + this.command.getName());
}
if (result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
throw new BindingException("Mapper method '" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
} else {
return result;
}
}
public Object convertArgsToSqlCommandParam(Object[] args) {
return this.paramNameResolver.getNamedParams(args);
}
ParamNameResolver类， 参数会保存成这样的格式{id:1,name:Tom,2:30,param1:1,param2:Tom,param3:30}
public class ParamNameResolver {
private static final String GENERIC_NAME_PREFIX = "param";
private final SortedMap<Integer, String> names;
private boolean hasParamAnnotation;
// 构造器来初始化names,其值是{0=id, 1=name, 2=2}
public ParamNameResolver(Configuration config, Method method) {
Class<?>[] paramTypes = method.getParameterTypes();
Annotation[][] paramAnnotations = method.getParameterAnnotations();
SortedMap<Integer, String> map = new TreeMap();
int paramCount = paramAnnotations.length; for(int paramIndex = 0; paramIndex < paramCount; ++paramIndex) {
if (!isSpecialParameter(paramTypes[paramIndex])) {
String name = null;
Annotation[] var9 = paramAnnotations[paramIndex];
int var10 = var9.length; for(int var11 = 0; var11 < var10; ++var11) {
Annotation annotation = var9[var11];
if (annotation instanceof Param) { // 如果使用了param注解的参数，例如@Param("id")
this.hasParamAnnotation = true;
name = ((Param)annotation).value(); // name = "id";
break;
}
} if (name == null) {
if (config.isUseActualParamName()) { // 如果配置文件中配置了useActualParamName=true
//允许使用方法签名中的名称作为语句参数名称。 为了使用该特性，你的项目必须采用 Java 8 编译，并且加上 -parameters 选项。（新增于 3.4.1）
name = this.getActualParamName(method, paramIndex); // name=参数名称（id）
} if (name == null) {
name = String.valueOf(map.size()); // name=当前map的索引 name=2
}
} map.put(paramIndex, name);
}
} this.names = Collections.unmodifiableSortedMap(map);
} private String getActualParamName(Method method, int paramIndex) {
return (String)ParamNameUtil.getParamNames(method).get(paramIndex);
} private static boolean isSpecialParameter(Class<?> clazz) {
return RowBounds.class.isAssignableFrom(clazz) || ResultHandler.class.isAssignableFrom(clazz);
} public String[] getNames() {
return (String[])this.names.values().toArray(new String[0]);
}
public Object getNamedParams(Object[] args) { // args【1，"Tom",30】
// names：{0=id, 1=name, 2=2}；构造器的时候就确定好了
int paramCount = this.names.size();
if (args != null && paramCount != 0) {
if (!this.hasParamAnnotation && paramCount == 1) { // 如果只有一个元素，并且没有Param注解；
return args[(Integer)this.names.firstKey()]; // 直接返回args[0]
} else { // 多个元素或者有Param标注
Map<String, Object> param = new ParamMap();
int i = 0; for(Iterator var5 = this.names.entrySet().iterator(); var5.hasNext(); ++i) {
Entry<Integer, String> entry = (Entry)var5.next();
//names集合的value作为key; names集合的key又作为取值的参考args[0]
//eg:{id=args[0]:1,name=args[1]:Tom,2=args[2]:30}
param.put((String)entry.getValue(), args[(Integer)entry.getKey()]);
//额外的将每一个参数也保存到map中，使用新的key：param1...paramN
//效果：有Param注解可以#{指定的key}，或者#{param1}
String genericParamName = "param" + String.valueOf(i + 1);
if (!this.names.containsValue(genericParamName)) {
param.put(genericParamName, args[(Integer)entry.getKey()]);
}
} return param;
}
} else {
return null;
}
}
}