自定义注解获取属性名

1 背景 对于以下代码, 小组内存在分歧

@Override public List<Student> getByParam(Student student) { return this.selectList( new EntityWrapper<Student>() .eq("student_name", student.studentName) .eq("student_age", student.studentAge); }

2 定义工具类实现

/** * 直接通过 fields[index]获取存在问题: 实体属性可能增减, 顺序可能打乱, 不可靠 */ Field[] fields = clazz.getDeclaredFields();

2.1 注解, 实体类

package com.***.***; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) public @interface MyAnnotation { int index() default 0; } ---------------------------------------------- package com.***.***; public class Student { @MyAnnotation(index = 1) private String studentName; @MyAnnotation(index = 2) private Integer studentAge; public Student() { } // getter and setter }

2.2 注解效果测试, 输出

@Test public void fun15() throws ClassNotFoundException { Class<?> clazz = Class.forName("com.***.***.Student"); Field[] fields = clazz.getDeclaredFields(); for (Field field : fields) { if (field.getAnnotation(MyAnnotation.class).index() == 1) { String s = field.toString(); System.out.println(s); // private java.lang.String com.***.***.Student.studentName System.out.println(s.substring(s.lastIndexOf(".") + 1)); // studentName } } }

3 实体属性转数据库字段的下划线格式

@Test public void fun16() { String s = "studentName"; String string = this.getString(s); System.out.println(string); // student_name } private String getString(String s) { if (StringUtils.isBlank(s)) { return ""; } char[] chars = s.toCharArray(); StringBuilder sb = new StringBuilder(); char low, c; for (int i = 0; i < chars.length; i++) { c = chars[i]; if (Character.isUpperCase(c)) { sb.append("_").append(Character.toLowerCase(c)); continue; } sb.append(c); } return sb.toString(); }

3.1 测试及输出

@Test public void fun17() throws Exception { String name = this.fieldToHump(Class.forName("com.***.***.Student"), 1); System.out.println(getString(name)); // student_name } // 获取指定属性(index: 想要的属性对应的注解值) private String fieldToHump(Class clazz, int index) throws Exception { Field[] fields = clazz.getDeclaredFields(); String s; for (Field field : fields) { if (field.getAnnotation(MyAnnotation.class).index() == index) { s = field.toString(); return s.substring(s.lastIndexOf(".") + 1); } } throw new RuntimeException("Are u stupid"); }

3.2 替代方案

// 原先代码中有很多这种字符串 @Override public List<Student> getByParam(Student student) { return this.selectList( new EntityWrapper<Student>() .eq("student_name", student.studentName) .eq("student_age", student.studentAge); } ------------------------------------------------------------ // 可以写成 @Override public List<Student> getByParam(Student student) { return this.selectList( new EntityWrapper<Student>() .eq(getString(fieldToHump(student.getClass(), 1)), studentName) .eq(getString(fieldToHump(student.getClass(), 2)), studentAge); }

4 其实已经存在解决方案(lombok版本有要求)

package com.***.***; import lombok.experimental.FieldNameConstants; @FieldNameConstants public class Student { private String studentName; private Integer studentAge; public Student() { } // getter and setter } ------------------------------------------------------------ @Test public void test18() { String studentName = Student.Fields.studentName; String studentAge = Student.Fields.studentAge; System.out.println(studentName); // studentName System.out.println(studentAge); // studentAge } ------------------------------------------------------------ <!--lombok支持 --> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <version>1.18.12</version> <scope>provided</scope> </dependency>