Java 註解和反射(六)獲取泛型,註解資訊
阿新 • • 發佈:2022-11-29
反射操作泛型
**Java採用泛型擦出的機制來引入泛型,Java中的泛型僅僅是給編譯器javac使用的,確保資料
的安全性和免除強制型別轉換問題,但是,一旦編譯完成,所有和泛型有關的型別全部擦除
**為了通過反射操作這些型別,Java新增了ParameterizedType,GenericArrayType,TypeVariable
和WildcardType幾種型別來代表不能被歸一到Class類中的型別但是又和原始型別齊名的型別
**ParmeterizedType:表示一種引數化型別,比如Collection<String>
**GenericArrayType:表示一種元素型別是引數化型別或者型別變數的陣列型別
**TypeVarible:是各種型別變數的公共父介面
**WildcardType:代表一種萬用字元型別表示式
public class Test10 { public void test01(Map<String,User> map , List<User> list){ System.out.println("test01"); } public Map<String,User> test02(){ System.out.println("test02"); return null; } public static void main(String[] args) throws NoSuchMethodException { Method method = Test10.class.getMethod("test01", Map.class, List.class); Type[] genericParameterTypes = method.getGenericParameterTypes(); for (Type genericParameterType : genericParameterTypes) { System.out.println("#"+genericParameterType); if (genericParameterType instanceof ParameterizedType){ Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments(); for (Type actualTypeArgument : actualTypeArguments) { System.out.println(actualTypeArgument); } } } method = Test10.class.getMethod("test02", null); Type genericReturnType = method.getGenericReturnType(); if (genericReturnType instanceof ParameterizedType){ Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments(); for (Type actualTypeArgument : actualTypeArguments) { System.out.println(actualTypeArgument); } } } }
反射操作註解
getAnnotations
getAnnotation
練習ORM
瞭解什麼是ORM?
Object relationship Mapping --> 物件關係對映
類和表結構對應
屬性和欄位對應
物件和記錄對應
要求:利用註解和反射完成類和表結構的對映關係
//練習反射操作註解 public class Test11 { public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException { Class c1 = Class.forName("refection.Student"); //通過反射獲得註解 Annotation[] annotations = c1.getAnnotations(); for (Annotation annotation : annotations) { System.out.println(annotation); } //獲得註解Value的值 TableLuo tableLuo = (TableLuo) c1.getAnnotation(TableLuo.class); String value = tableLuo.value(); System.out.println(value); //獲得屬性的註解 Field name = c1.getDeclaredField("name"); FieldLuo annotations1 = name.getAnnotation(FieldLuo.class); System.out.println(annotations1.columnName()); System.out.println(annotations1.length()); System.out.println(annotations1.type()); } } @TableLuo("db_student") class Student{ @FieldLuo(columnName = "db_id",type = "int",length = 10) private int id; @FieldLuo(columnName = "db_age",type = "int",length = 10) private int age; @FieldLuo(columnName = "db_name",type = "varchar",length = 3) private String name; public Student() { } public Student(int id, int age, String name) { this.id = id; this.age = age; this.name = name; } public int getId() { return id; } public void setId(int id) { this.id = id; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "student{" + "id=" + id + ", age=" + age + ", name='" + name + '\'' + '}'; } } //類名註解 @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @interface TableLuo{ String value(); } //屬性註解 @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) @interface FieldLuo{ String columnName(); String type(); int length(); }