1、实现功能
我们已经对注解有了基本的认识，知道了如何自定义注解，如何使用和最基本的处理注解。
本篇主要介绍，如何使用运行时级别的注解，配合反射来自动生成建表的sql语句。如下例：
我们有实体类Student，并添加相应的注解 @Table、@Column，最终可以获取到创建对应表的sql语句
@Table(name = "t_student")
public class Student {
@Column(name = "c_name")
public String name;
@Column(name = "c_sex")
public String sex;
...
}11
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@Table(name = "t_student")
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public class Student {
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@Column(name = "c_name")
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public String name;
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@Column(name = "c_sex")
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public String sex;
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...
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}
2、自定义注解
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
//设置表名
String name();
}
@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
//设置字段名
String name();
}11
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@Retention(RetentionPolicy.RUNTIME)
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public @interface Table {
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//设置表名
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String name();
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}
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@Retention(RetentionPolicy.RUNTIME)
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public @interface Column {
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//设置字段名
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String name();
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}
3、对实体类使用注解
/**
* 学生类
*/
@Table(name = "t_student")
public class Student {
@Column(name = "c_name")
public String name;
@Column(name = "c_sex")
public String sex;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
}28
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/**
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* 学生类
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*/
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@Table(name = "t_student")
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public class Student {
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@Column(name = "c_name")
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public String name;
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@Column(name = "c_sex")
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public String sex;
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public String getSex() {
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return sex;
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}
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public void setSex(String sex) {
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this.sex = sex;
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}
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}
4、运行时获取注解并转化
4.1 获取表名
private static String getTableName(Class<?> clazz) {
String name = null;
//如果有@Table注解
if (clazz.isAnnotationPresent(Table.class)) {
Table table = clazz.getAnnotation(Table.class);
name = table.name();
}
return name;
}10
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private static String getTableName(Class<?> clazz) {
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String name = null;
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//如果有@Table注解
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if (clazz.isAnnotationPresent(Table.class)) {
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Table table = clazz.getAnnotation(Table.class);
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name = table.name();
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}
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return name;
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}
4.2 获取字段名和类型
4.2.1 建立NameAndType类用来封装获取到的字段名和类型
public class NameAndType {
String name;
String type;
public NameAndType(String name, String type) {
this.name = name;
this.type = type;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
}27
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public class NameAndType {
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String name;
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String type;
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public NameAndType(String name, String type) {
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this.name = name;
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this.type = type;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public String getType() {
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return type;
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}
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public void setType(String type) {
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this.type = type;
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}
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}
4.2.2 获取字段名和类型
private static List<NameAndType> getColumns(Class<?> clazz) throws Exception {
List<NameAndType> colums = new ArrayList<NameAndType>();
Field[] fields = clazz.getDeclaredFields();
if (fields != null) {
//分析clazz中变量是否需要生成sql字段
for (Field field : fields) {
if (field.isAnnotationPresent(Column.class)) {
//生成sql字段的字段名
Column column = field.getAnnotation(Column.class);
String name = column.name();
//生成sql字段的字段类型
String type;
if (int.class.isAssignableFrom(field.getType())) {
type = "integer";
}
else if (String.class.isAssignableFrom(field.getType())) {
type = "text";
}
else {
throw new Exception("unsupported type");
}
colums.add(new NameAndType(name, type));
}
}
}
return colums;
}28
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private static List<NameAndType> getColumns(Class<?> clazz) throws Exception {
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List<NameAndType> colums = new ArrayList<NameAndType>();
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Field[] fields = clazz.getDeclaredFields();
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if (fields != null) {
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//分析clazz中变量是否需要生成sql字段
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for (Field field : fields) {
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if (field.isAnnotationPresent(Column.class)) {
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//生成sql字段的字段名
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Column column = field.getAnnotation(Column.class);
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String name = column.name();
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//生成sql字段的字段类型
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String type;
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if (int.class.isAssignableFrom(field.getType())) {
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type = "integer";
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}
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else if (String.class.isAssignableFrom(field.getType())) {
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type = "text";
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}
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else {
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throw new Exception("unsupported type");
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}
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colums.add(new NameAndType(name, type));
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}
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}
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}
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return colums;
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}
注意Class类的isAssignableFrom方法可以判断两个类是不是存在父子关系。
4.3 生成建表sql
public static String createTable(Class<?> clazz) throws Exception {
String sql = null;
String tableName = getTableName(clazz);
List<NameAndType> columns = getColumns(clazz);
if (tableName != null && !tableName.equals("") && !columns.isEmpty()) {
StringBuffer strBuffer = new StringBuffer("CREATE TABLE " + tableName + "( ");
for (NameAndType column : columns) {
String bean = column.getName() + " " + column.getType() + ", ";
strBuffer.append(bean);
}
//删除最后多余的一个逗号
strBuffer.deleteCharAt(strBuffer.length() - 2);
strBuffer.append(");");
sql = strBuffer.toString();
}
return sql;
}19
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public static String createTable(Class<?> clazz) throws Exception {
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String sql = null;
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String tableName = getTableName(clazz);
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List<NameAndType> columns = getColumns(clazz);
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if (tableName != null && !tableName.equals("") && !columns.isEmpty()) {
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StringBuffer strBuffer = new StringBuffer("CREATE TABLE " + tableName + "( ");
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for (NameAndType column : columns) {
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String bean = column.getName() + " " + column.getType() + ", ";
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strBuffer.append(bean);
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}
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//删除最后多余的一个逗号
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strBuffer.deleteCharAt(strBuffer.length() - 2);
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strBuffer.append(");");
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sql = strBuffer.toString();
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}
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return sql;
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}
4.4 测试和结果
public class TestAnnotation {
public static void main(String[] args) throws Exception {
String sql = TableAnnotationUtil.createTable(Student.class);
System.out.println(sql);
}
}
//结果输出
CREATE TABLE t_student( c_name text, c_sex text );x
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public class TestAnnotation {
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public static void main(String[] args) throws Exception {
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String sql = TableAnnotationUtil.createTable(Student.class);
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System.out.println(sql);
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}
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}
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//结果输出
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CREATE TABLE t_student( c_name text, c_sex text );