Java Annotaion认识

本文深入解析Java注解(annotation)的概念及应用,通过实例演示如何利用注解定义数据库表结构,并通过反射机制生成对应的SQL创建语句。

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Annotation是现在大部分框架都在使用的技术,并且Java官方也在5.0版本加入支持;
以前做项目时候,经常接触到这个annotation技术,但就是不理解原理,
今天学习了,总结如下:
annotation是对类、属性、方法、参数等而外增加的限制条件,通过Java的反射机制,
读取这些对象的注释(annotation),就可以更好的控制使用这些对象,
从而解耦了操作逻辑和业务逻辑,让我们更关注业务逻辑实现功能;
例子如下:

package com.funo.webreport.annotation;

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)
public @interface DBTable {
public String name() default "";
}

package com.funo.webreport.annotation;

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 Constraints {
public boolean primaryKey() default false;

public boolean allowNull() default true;

public boolean unique() default false;
}

package com.funo.webreport.annotation;

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 SQLString {
public int value() default 0;

public String name() default "";

public Constraints constraints() default @Constraints;
}

package com.funo.webreport.annotation;

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 SQLInt {
public String name() default "";
public Constraints constraints() default @Constraints;
}

package com.funo.webreport.annotation;

@DBTable()
public class User {

@SQLString(value = 32, name = "u_id",
constraints = @Constraints(
primaryKey = true, unique = true, allowNull = false))
private String id;

@SQLString(value = 10, name = "u_name")
private String name;

@SQLString(value = 30, name = "u_password")
private String password;

@SQLInt(name = "u_age")
private int age;

public void setId(String id) {
this.id = id;
}

public String getId() {
return this.id;
}

public void setName(String name) {
this.name = name;
}

public String getName() {
return this.name;
}

public void setPassword(String password) {
this.password = password;
}

public String getPassword() {
return this.password;
}

public void setAge(int age) {
this.age = age;
}

public int getAge() {
return this.age;
}
}

package com.funo.webreport.annotation;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class TestFrame {

public static void main(String[] args) {
Class userClass = User.class;
DBTable dbTable = (DBTable) userClass.getAnnotation(DBTable.class);
String dbTableName = userClass.getSimpleName();
if (dbTable == null) {
return;
}
if (dbTable.name().length() > 0) {
dbTableName = dbTable.name().toUpperCase();
}
StringBuffer createSql = new StringBuffer("CREATE TABLE " + dbTableName + "(");
Field[] fields = userClass.getDeclaredFields();
List<String> colDef = new ArrayList<String>();
for (Field field : fields) {
Annotation[] annotions = field.getAnnotations();
if (annotions.length < 1) {
continue;
}
for (Annotation anno : annotions) {
String colName = field.getName().toUpperCase();
if (anno instanceof SQLString) {
SQLString sqlStr = (SQLString) anno;
if (sqlStr.name().length() > 0) {
colName = sqlStr.name().toUpperCase();
}
int len = sqlStr.value();
colDef.add(colName + " " + "varchar(" + len + ")" + getConstraints(sqlStr.constraints()));
} else if (anno instanceof SQLInt) {
SQLInt sqlInt = (SQLInt) anno;
if (sqlInt.name().length() > 0) {
colName = sqlInt.name().toUpperCase();
}
colDef.add(colName + " " + " int " + getConstraints(sqlInt.constraints()));
}
}
}
Iterator<String> it = colDef.iterator();
while (it.hasNext()) {
createSql.append("\n " + it.next() + ",");
}
String cmd = createSql.substring(0, createSql.length() - 1);
cmd += ");";
System.out.println(cmd);
}

private static String getConstraints(Constraints constraints) {
String cons = "";
if (!constraints.allowNull()) {
cons += " not null ";
}
if (constraints.primaryKey()) {
cons += " primary key ";
}
if (constraints.unique()) {
cons += " unique ";
}
return cons;
}
}

输出结果:

CREATE TABLE User(
U_ID varchar(32) not null primary key unique ,
U_NAME varchar(10),
U_PASSWORD varchar(30),
U_AGE int );

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