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+# mybatis 反射
+
+## addDefaultConstructor
+
+-  mybatis 的反射相关内容在`org.apache.ibatis.reflection` 下存放. 本片主要讲解`org.apache.ibatis.reflection.Reflector`类, 先看一下该类的属性
+```java
+public class Reflector {
+
+     /**
+        * 实体类.class
+        */
+       private final Class<?> type;
+       /**
+        * 可读 属性
+        */
+       private final String[] readablePropertyNames;
+       /**
+        * 可写 属性值
+        */
+       private final String[] writablePropertyNames;
+       /**
+        * set 方法列表
+        */
+       private final Map<String, Invoker> setMethods = new HashMap<>();
+       /**
+        * get 方法列表
+        */
+       private final Map<String, Invoker> getMethods = new HashMap<>();
+       /**
+        * set 的数据类型
+        */
+       private final Map<String, Class<?>> setTypes = new HashMap<>();
+       /**
+        * get 的数据类型
+        */
+       private final Map<String, Class<?>> getTypes = new HashMap<>();
+       /**
+        * 构造函数
+        */
+       private Constructor<?> defaultConstructor;
+   
+       /**
+        * 缓存数据, 大写KEY
+        */
+       private Map<String, String> caseInsensitivePropertyMap = new HashMap<>();
+
+}
+```
+
+- 构造方法, 构造方法传入一个类的字节码,在构造方法中设置相关的属性值
+```java
+public class Reflector {
+
+/**
+ * @param clazz 待解析类的字节码
+ */
+public Reflector(Class<?> clazz) {
+    type = clazz;
+    // 构造方法
+    addDefaultConstructor(clazz);
+    // get 方法
+    addGetMethods(clazz);
+    // set 方法
+    addSetMethods(clazz);
+    // 字段值
+    addFields(clazz);
+    readablePropertyNames = getMethods.keySet().toArray(new String[0]);
+    writablePropertyNames = setMethods.keySet().toArray(new String[0]);
+    for (String propName : readablePropertyNames) {
+        // 循环操作设置到缓存中,
+        caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
+    }
+    for (String propName : writablePropertyNames) {
+        caseInsensitivePropertyMap.put(propName.toUpperCase(Locale.ENGLISH), propName);
+    }
+}
+}
+```
+- `addDefaultConstructor` 方法 , 下面截图内容为JDK8 mybatis中 的内容
+```java
+    private void addDefaultConstructor(Class<?> clazz) {
+
+        // 获取类里面的所有构造方法
+        Constructor<?>[] constructors = clazz.getDeclaredConstructors();
+        // 过滤得到空参构造 constructor -> constructor.getParameterTypes().length == 0
+        Arrays.stream(constructors).filter(constructor -> constructor.getParameterTypes().length == 0)
+                .findAny().ifPresent(constructor -> {
+            System.out.println("有空参构造");
+            this.defaultConstructor = constructor;
+        });
+    }
+```
+- 创建一个测试类
+```java 
+public class People {
+    private String name;
+
+    public People() {
+    }
+
+    public People(String name) {
+        this.name = name;
+    }
+
+    @Override
+    public String toString() {
+        return "People{" +
+                "name='" + name + '\'' +
+                '}';
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+}
+
+```
+```java
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Constructor;
+
+class HfReflectorTest {
+    @Test
+    void getDefaultConstructorTest() throws Exception {
+        Reflector reflector = new Reflector(People.class);
+        // 获取空参构造方法
+        Constructor<?> defaultConstructor = reflector.getDefaultConstructor();
+        People o = (People) defaultConstructor.newInstance();
+        o.setName("hhh");
+
+        System.out.println(o);
+    }
+}
+```
+
+- 准备工作完成了开始进行 debug , 在`org.apache.ibatis.reflection.Reflector#addDefaultConstructor`这个方法上打上断点
+
+  ![1575890354400](/images/mybatis/1575890354400.png)
+
+  观察`constructors`属性存在两个方法,这两个方法就是我在`People`类中的构造方法.
+
+  根据语法内容我们应该对`parameterTypes`属性进行查看
+
+  ![1575890475839](/images/mybatis/1575890475839.png)
+
+可以发现空参构造的`parameterTypes`长度是0.因此可以确认`org.apache.ibatis.reflection.Reflector#addDefaultConstructor`方法获取了空参构造
+
+- 继续看`org.apache.ibatis.reflection.Reflector#getDefaultConstructor`方法, 该方法是获取构造函数的方法,如果构造函数没有就抛出异常,这也是为什么我们的实体类需要把空参构造写上去的原因。
+
+  ```java
+      public Constructor<?> getDefaultConstructor() {
+          if (defaultConstructor != null) {
+              return defaultConstructor;
+          } else {
+              // 如果没有空参构造抛出的异常
+              throw new ReflectionException("There is no default constructor for " + type);
+          }
+      }
+  ```
+
+  
+
+
+
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