注解

注解为我们在代码中添加信息提供了形式化的方法, 使我们可以在稍后某个时刻非常方便的使用这些元素.

Java支持的三种标准注解

• @Override
• @Deprecated
• @Suppress Warnings

Java支持的四种元注解

• @Targe

  表示该注解可以用于什么地方.可能的ElementType参数包括

  • CONSTRUCTOR 构造方法
  • FIELD 成员变量
  • LOCAL_VARIABLE 局部变量
  • METHOD 方法
  • PACKAGE 包
  • PAPAMETER 参数
  • TYPE 类, 接口, 枚举

• @Retention riˈtenCHən

  • SOURCE 注解将被编译器丢弃
  • CLASS 注解在class文件中可用, 但是会被VM丢弃
  • RUNTIME VM 在运行期间也保留注解, 可以通过反射机制来获取注解的信息

• @Documented

  将该注解包含在javadoc 中

• @Inherited

  允许子类继承父类中的注解

注解元素

注解元素可以用的类型如下所示

• 所有基本类型 int, long, float, double, short, byte, boolean, void
• String
• Class
• enum
• Annotation
• 以上类型的数组

生成外部文件

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface DbTable {
    public String name() default "";
}
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface ConStraints {
    boolean primaryKey() default false;
    boolean allowNull() default true;
    boolean unqiue() default false;
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface SQLString {
    int value() default 0;
    String name() default "";
    //
    ConStraints conStraints() default @ConStraints;
}
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface SQLInter {
    String name() default "";
    ConStraints conStraints() default @ConStraints;
}

在使用的时候

@DbTable(name = "member")
@Date
public class Member {
    @SQLString(30)
    private String firstName;
    @SQLString(50)
    private String lastName;
    @SQLInter
    private Integer age;
    @SQLString(value = 30, conStraints = @ConStraints(primaryKey = true))
    private String handler;
}

框架是通过类似这样的方法来解析的

public class Main {
    public static void main(String[] args) {
        Class member = Member.class;
        DbTable dbTable = member.getAnnotationsByType(DbTable.class);
        if (! Objects.isNull(dbTable)) {
            String tableName = dbTable.name();
            if (! Objects.isNull(tableName) && tableName.length() > 0) {
                tableName = tableName.toUpperCase();
            } else {
                tableName = member.getName().toUpperCase();
            }
            Field[] fields = member.getDeclaredFields();
            StringBuilder[] fieldsDefs = new StringBuilder[fields.length];

            for (int i = 0; i < fields.length; i ++) {
                Field field = fields[i];
                fieldsDef[i] = new StringBuilder();

                Annotation[] annotations = field.getAnnotations();
                if (annotations.length == 0) {
                    continue;
                }
                for (Annotation annotation : annotations) {
                    String columnName = "";
                    if (annotation instanceof SQLInter) {
                        SQLInter sqlInter = (SQLInter) annotation;
                        fieldsDef[i].append(" INT ").append( getConstrains(sqlInter.conStraints()));
                    }
                }
                // many many other type
            }

            StringBuilder createCommand = new StringBuilder();
            createCommand.append("crate table ").append(tableName).append(" (");
            for (StringBuilder stringBuilder : fieldsDefs ) {
                createCommand.append(stringBuilder).append(" , ");
            }
            // 去掉最后一个逗号
            createCommand = createCommand.subSequence(0, createCommand.length() - 2);
            createCommand.append(" );");             
        }
    }

    private static String getConstrains(ConStraints con) {
        StringBuilder constraints = new StringBuilder();
        if (! con.allowNull()) {
            constraints.append(" NOT NUll ");
        }

        if (con.primaryKey()) {
            constraints.append(" PRIMARY KEY ");
        }

        if (con.unqiue()) {
            constraints.append(" UNIQUE ");
        }

        return constraints.toString();
    }
}