本文节选自《设计模式就该这样学》
1 建造者模式的链式写法
以构建一门课程为例，一个完整的课程由PPT课件、回放视频、课堂笔记、课后作业组成，但是这些内容的设置顺序可以随意调整，我们用建造者模式来代入理解一下。首先创建一个产品类Course。
代码语言：txt复制@Data
public class Course {
private String name;
private String ppt;
private String video;
private String note;
private String homework;
@Override
public String toString() {
return "CourseBuilder{"
"name='" name '''
", ppt='" ppt '''
", video='" video '''
", note='" note '''
", homework='" homework '''
'}';
}
}然后创建建造者类CourseBuilder，将复杂的创建过程封装起来，创建步骤由用户决定。
代码语言：txt复制public class CourseBuilder {
private Course course = new Course();
public CourseBuilder addName(String name){
course.setName(name);
return this;
}
public CourseBuilder addPpt(String ppt){
course.setPpt(ppt);
return this;
}
public CourseBuilder addVideo(String video){
course.setVideo(video);
return this;
}
public CourseBuilder addNote(String note){
course.setNote(note);
return this;
}
public CourseBuilder addHomework(String homework){
course.setHomework(homework);
return this;
}
public Course builder(){
return course;
}
}最后编写客户端测试代码。
代码语言：txt复制 public static void main(String[] args) {
CourseBuilder builder = new CourseBuilder()
.addName("设计模式")
.addPPT("【PPT课件】")
.addVideo("【回放视频】")
.addNote("【课堂笔记】")
.addHomework("【课后作业】");
System.out.println(builder.build());
}这样的写法是不是很眼熟？后面分析建造者模式在框架源码中的应用时大家就会明白。再来看一下类图的变化，如下图所示。
image.png2 使用静态内部类实现建造者模式
事实上，在平常的编码中，我们通常都会忽略对象的复杂性，优先考虑使用工厂模式创建对象，而不是建造者模式。因为工厂模式和建造者模式的作用都是创建一个产品对象，而工厂模式的结构更加简洁直接（没有Builder和 Director），因此更常使用。
一般情况下，我们更习惯使用静态内部类的方式实现建造者模式，即一个产品类内部自动带有一个具体建造者，由它负责该产品的组装创建，不再需要Builder和Director，这样，产品表示与创建之间的联系更加紧密，结构更加紧凑，同时使得建造者模式的形式更加简洁。
如果采用静态内部类形式实现建造者模式，则前面的案例可以改写如下。
代码语言：txt复制@Data
public class Course {
private String name;
private String ppt;
private String video;
private String note;
private String homework;
@Override
public String toString() {
return "Course{"
"name='" name '''
", ppt='" ppt '''
", video='" video '''
", note='" note '''
", homework='" homework '''
'}';
}
public static class Builder {
private Course course = new Course();
public Builder addName(String name){
course.setName(name);
return this;
}
public Builder addPpt(String ppt){
course.setPpt(ppt);
return this;
}
public Builder addVideo(String video){
course.setVideo(video);
return this;
}
public Builder addNote(String note){
course.setNote(note);
return this;
}
public Builder addHomework(String homework){
course.setHomework(homework);
return this;
}
public Course builder(){
return course;
}
}
}客户端测试代码如下。
代码语言：txt复制 public static void main(String[] args) {
Course course = new Course.Builder()
.addName("设计模式")
.addPpt("【PPT课件】")
.addVideo("【录播视频】")
.builder();
System.out.println(course);
}
这样，代码也会看上去更加简洁，不会让人感觉到多了一个类。
3 使用建造者模式动态构建SQL语句
下面来看一个实战案例，这个案例参考了开源框架JPA的SQL构造模式。我们在构造SQL查询条件的时候，需要根据不同的条件来拼接SQL字符串。如果查询条件复杂，则SQL拼接的过程也会变得非常复杂，从而给代码维护带来非常大的困难。因此，我们用建造者类QueryRuleSqlBuilder将复杂的SQL构造过程进行封装，用QueryRule对象专门保存SQL查询时的条件，最后根据查询条件，自动生成SQL语句。首先创建QueryRule类，代码如下。
代码语言：txt复制import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
/**
* QueryRule，主要功能用于构造查询条件
*
* @author Tom
*/
public final class QueryRule implements Serializable
{
private static final long serialVersionUID = 1L;
public static final int ASC_ORDER = 101;
public static final int DESC_ORDER = 102;
public static final int LIKE = 1;
public static final int IN = 2;
public static final int NOTIN = 3;
public static final int BETWEEN = 4;
public static final int EQ = 5;
public static final int NOTEQ = 6;
public static final int GT = 7;
public static final int GE = 8;
public static final int LT = 9;
public static final int LE = 10;
public static final int ISNULL = 11;
public static final int ISNOTNULL = 12;
public static final int ISEMPTY = 13;
public static final int ISNOTEMPTY = 14;
public static final int AND = 201;
public static final int OR = 202;
private List<Rule> ruleList = new ArrayList<Rule>();
private List<QueryRule> queryRuleList = new ArrayList<QueryRule>();
private String propertyName;
private QueryRule() {}
private QueryRule(String propertyName) {
this.propertyName = propertyName;
}
public static QueryRule getInstance() {
return new QueryRule();
}
/**
* 添加升序规则
* @param propertyName
* @return
*/
public QueryRule addAscOrder(String propertyName) {
this.ruleList.add(new Rule(ASC_ORDER, propertyName));
return this;
}
/**
* 添加降序规则
* @param propertyName
* @return
*/
public QueryRule addDescOrder(String propertyName) {
this.ruleList.add(new Rule(DESC_ORDER, propertyName));
return this;
}
public QueryRule andIsNull(String propertyName) {
this.ruleList.add(new Rule(ISNULL, propertyName).setAndOr(AND));
return this;
}
public QueryRule andIsNotNull(String propertyName) {
this.ruleList.add(new Rule(ISNOTNULL, propertyName).setAndOr(AND));
return this;
}
public QueryRule andIsEmpty(String propertyName) {
this.ruleList.add(new Rule(ISEMPTY, propertyName).setAndOr(AND));
return this;
}
public QueryRule andIsNotEmpty(String propertyName) {
this.ruleList.add(new Rule(ISNOTEMPTY, propertyName).setAndOr(AND));
return this;
}
public QueryRule andLike(String propertyName, Object value) {
this.ruleList.add(new Rule(LIKE, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule andEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(EQ, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule andBetween(String propertyName, Object... values) {
this.ruleList.add(new Rule(BETWEEN, propertyName, values).setAndOr(AND));
return this;
}
public QueryRule andIn(String propertyName, List<Object> values) {
this.ruleList.add(new Rule(IN, propertyName, new Object[] { values }).setAndOr(AND));
return this;
}
public QueryRule andIn(String propertyName, Object... values) {
this.ruleList.add(new Rule(IN, propertyName, values).setAndOr(AND));
return this;
}
public QueryRule andNotIn(String propertyName, List<Object> values) {
this.ruleList.add(new Rule(NOTIN,
propertyName,
new Object[] { values }).setAndOr(AND));
return this;
}
//此处省略部分代码
public List<Rule> getRuleList() {
return this.ruleList;
}
public List<QueryRule> getQueryRuleList() {
return this.queryRuleList;
}
public String getPropertyName() {
return this.propertyName;
}
protected class Rule implements Serializable {
private static final long serialVersionUID = 1L;
private int type; //规则的类型
private String property_name;
private Object[] values;
private int andOr = AND;
public Rule(int paramInt, String paramString) {
this.property_name = paramString;
this.type = paramInt;
}
public Rule(int paramInt, String paramString,
Object[] paramArrayOfObject) {
this.property_name = paramString;
this.values = paramArrayOfObject;
this.type = paramInt;
}
public Rule setAndOr(int andOr){
this.andOr = andOr;
return this;
}
public int getAndOr(){
return this.andOr;
}
public Object[] getValues() {
return this.values;
}
public int getType() {
return this.type;
}
public String getPropertyName() {
return this.property_name;
}
}
}然后创建QueryRuleSqlBuilder类。
代码语言：txt复制package com.tom.vip.pattern.builder.sql;
/**
* 根据QueryRule自动构建SQL语句
* @author Tom
*
*/
public class QueryRuleSqlBuilder {
private int CURR_INDEX = 0; //记录参数所在的位置
private List<String> properties; //保存列名列表
private List<Object> values; //保存参数值列表
private List<Order> orders; //保存排序规则列表
private String whereSql = "";
private String orderSql = "";
private Object [] valueArr = new Object[]{};
private Map<Object,Object> valueMap = new HashMap<Object,Object>();
/**
* 获得查询条件
* @return
*/
private String getWhereSql(){
return this.whereSql;
}
/**
* 获得排序条件
* @return
*/
private String getOrderSql(){
return this.orderSql;
}
/**
* 获得参数值列表
* @return
*/
public Object [] getValues(){
return this.valueArr;
}
/**
* 获得参数列表
* @return
*/
private Map<Object,Object> getValueMap(){
return this.valueMap;
}
/**
* 创建SQL构造器
* @param queryRule
*/
public QueryRuleSqlBuilder(QueryRule queryRule) {
CURR_INDEX = 0;
properties = new ArrayList<String>();
values = new ArrayList<Object>();
orders = new ArrayList<Order>();
for (QueryRule.Rule rule : queryRule.getRuleList()) {
switch (rule.getType()) {
case QueryRule.BETWEEN:
processBetween(rule);
break;
case QueryRule.EQ:
processEqual(rule);
break;
case QueryRule.LIKE:
processLike(rule);
break;
case QueryRule.NOTEQ:
processNotEqual(rule);
break;
case QueryRule.GT:
processGreaterThen(rule);
break;
case QueryRule.GE:
processGreaterEqual(rule);
break;
case QueryRule.LT:
processLessThen(rule);
break;
case QueryRule.LE:
processLessEqual(rule);
break;
case QueryRule.IN:
processIN(rule);
break;
case QueryRule.NOTIN:
processNotIN(rule);
break;
case QueryRule.ISNULL:
processIsNull(rule);
break;
case QueryRule.ISNOTNULL:
processIsNotNull(rule);
break;
case QueryRule.ISEMPTY:
processIsEmpty(rule);
break;
case QueryRule.ISNOTEMPTY:
processIsNotEmpty(rule);
break;
case QueryRule.ASC_ORDER:
processOrder(rule);
break;
case QueryRule.DESC_ORDER:
processOrder(rule);
break;
default:
throw new IllegalArgumentException("type" rule.getType() "not supported.");
}
}
//拼装where语句
appendWhereSql();
//拼装排序语句
appendOrderSql();
//拼装参数值
appendValues();
}
/**
* 去掉order
*
* @param sql
* @return
*/
private String removeOrders(String sql) {
Pattern p = Pattern.compile("order\s*by[\w|\W|\s|\S]*", Pattern.CASE_INSENSITIVE);
Matcher m = p.matcher(sql);
StringBuffer sb = new StringBuffer();
while (m.find()) {
m.appendReplacement(sb, "");
}
m.appendTail(sb);
return sb.toString();
}
/**
* 去掉select
*
* @param sql
* @return
*/
private String removeSelect(String sql) {
if(sql.toLowerCase().matches("from\s ")){
int beginPos = sql.toLowerCase().indexOf("from");
return sql.substring(beginPos);
}else{
return sql;
}
}
/**
* 处理like
* @param rule
*/
private void processLike(QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
Object obj = rule.getValues()[0];
if (obj != null) {
String value = obj.toString();
if (!StringUtils.isEmpty(value)) {
value = value.replace('*', '%');
obj = value;
}
}
add(rule.getAndOr(),rule.getPropertyName(),"like","%" rule.getValues()[0] "%");
}
/**
* 处理between
* @param rule
*/
private void processBetween(QueryRule.Rule rule) {
if ((ArrayUtils.isEmpty(rule.getValues()))
|| (rule.getValues().length < 2)) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),"","between",rule.getValues()[0],"and");
add(0,"","","",rule.getValues()[1],"");
}
//此处省略部分代码
/**
* 加入SQL查询规则队列
* @param andOr and或者or
* @param key 列名
* @param split 列名与值之间的间隔
* @param value 值
*/
private void add(int andOr,String key,String split ,Object value){
add(andOr,key,split,"",value,"");
}
/**
* 加入SQL查询规则队列
* @param andOr and或者or
* @param key 列名
* @param split 列名与值之间的间隔
* @param prefix 值前缀
* @param value 值
* @param suffix 值后缀
*/
private void add(int andOr,String key,String split ,String prefix,Object value,String suffix){
String andOrStr = (0 == andOr ? "" :(QueryRule.AND == andOr ? " and " : " or "));
properties.add(CURR_INDEX,
andOrStr key " " split prefix (null != value ? " ? " : " ") suffix);
if(null != value){
values.add(CURR_INDEX,value);
CURR_INDEX ;
}
}
/**
* 拼装where语句
*/
private void appendWhereSql(){
StringBuffer whereSql = new StringBuffer();
for (String p : properties) {
whereSql.append(p);
}
this.whereSql = removeSelect(removeOrders(whereSql.toString()));
}
/**
* 拼装排序语句
*/
private void appendOrderSql(){
StringBuffer orderSql = new StringBuffer();
for (int i = 0 ; i < orders.size(); i ) {
if(i > 0 && i < orders.size()){
orderSql.append(",");
}
orderSql.append(orders.get(i).toString());
}
this.orderSql = removeSelect(removeOrders(orderSql.toString()));
}
/**
* 拼装参数值
*/
private void appendValues(){
Object [] val = new Object[values.size()];
for (int i = 0; i < values.size(); i ) {
val[i] = values.get(i);
valueMap.put(i, values.get(i));
}
this.valueArr = val;
}
public String builder(String tableName){
String ws = removeFirstAnd(this.getWhereSql());
String whereSql = ("".equals(ws) ? ws : (" where " ws));
String sql = "select * from " tableName whereSql;
Object [] values = this.getValues();
String orderSql = this.getOrderSql();
orderSql = (StringUtils.isEmpty(orderSql) ? " " : (" order by " orderSql));
sql = orderSql;
return sql;
}
private String removeFirstAnd(String sql){
if(StringUtils.isEmpty(sql)){return sql;}
return sql.trim().toLowerCase().replaceAll("^\s*and", "") " ";
}
}接着创建Order类。
代码语言：txt复制/**
* SQL排序组件
* @author Tom
*/
public class Order {
private boolean ascending; //升序还是降序
private String propertyName; //哪个字段升序，哪个字段降序
public String toString() {
return propertyName ' ' (ascending ? "asc" : "desc");
}
/**
* Constructor for Order.
*/
protected Order(String propertyName, boolean ascending) {
this.propertyName = propertyName;
this.ascending = ascending;
}
/**
* Ascending order
*
* @param propertyName
* @return Order
*/
public static Order asc(String propertyName) {
return new Order(propertyName, true);
}
/**
* Descending order
*
* @param propertyName
* @return Order
*/
public static Order desc(String propertyName) {
return new Order(propertyName, false);
}
}最后编写客户端测试代码。
代码语言：txt复制public static void main(String[] args) {
QueryRule queryRule = QueryRule.getInstance();
queryRule.addAscOrder("age");
queryRule.andEqual("addr","Changsha");
queryRule.andLike("name","Tom");
QueryRuleSqlBuilder builder = new QueryRuleSqlBuilder(queryRule);
System.out.println(builder.builder("t_member"));
System.out.println("Params: " Arrays.toString(builder.getValues()));
}这样一来，客户端代码就非常清楚，运行结果如下图所示。
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