[TOC]
1. 定义
建造者模式(Builder Pattern)是将一个复杂对象的构建过程与它的表示分离,使得同样的构建过程可以创建不同的表示,属于创建型模式.使用建造者模式对于用户而言只需指定需要建造的类型就可以获得对象,建造过程及细节不需要了解
官方原文:Separate the construction of acomplex object from its representation so that the same construction process can create different representations
建造者模式适用于创建对象需要很多步骤,但是步骤的顺序不一定固定.如果一个对象有非常复杂的内部结构(很多属性),可以将复杂对象的创建和使用进行分离
建造者模式的设计中主要有四个角色:
- 产品(Product):要创建的产品类对象
- 建造者抽象(Builder):建造者的抽象类,规范产品对象的各个组成部分的建造,一般由子类实现具体的建造过程
- 建造者(ConcreteBuilder):具体的 Builder 类,根据不同的业务逻辑,具体化对象的各个组成部分的创建
- 调用者(Director):调用具体的建造者,来创建对象的各个部分,在指导者中不涉及具体产品的信息,只负责保证对象各部分完整创建或按某种顺序创建
2. 建造者模式的应用场景
建造者模式适用于一个具有较多的零件的复杂产品的创建过程,由于需求的变化,组成这个复杂产品的各个零件经常猛烈变化,但是它们的组合方式却相对稳定
建造者模式适用于以下几种场景:
- 相同的方法,不同的执行顺序,产生不同的结果时
- 多个部件或零件,都可以装配到一个对象中,但是产生的结果又不相同
- 产品类非常复杂,或者产品类中的调用顺序不同产生不同的作用
- 当初始化一个对象特别复杂,参数多,而且很多参数都具有默认值时
3. 建造者模式的基本写法
我们还是以课程为例,一个完整的课程需要由 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.首先我们创建一个需要构造
的产品类 Course:
@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,将复杂的构造过程封装起来,构造步骤由用户决定:
public class CourseBuilder{
private Course course = new Course();
public void addName(String name) {
course.setName(name);
}
public void addPPT(String ppt) {
course.setPpt(ppt);
}
public void addVideo(String video) {
course.setVideo(video);
}
public void addNote(String note) {
course.setNote(note);
}
public void addHomework(String homework) {
course.setHomework(homework);
}
public Course build() {
return course;
}
}
编写测试类:
public class Test {
public static void main(String[] args) {
CourseBuilder builder = new CourseBuilder();
builder.addName("设计模式");
builder.addPPT("【PPT 课件】");
builder.addVideo("【回放视频】");
builder.addNote("【课堂笔记】");
builder.addHomework("【课后作业】");
System.out.println(builder.build());
}
}
来看一下类结构图:
image
3. 建造者模式的链式写法
在平时的应用中,建造者模式通常是采用链式编程的方式构造对象,下面我们来一下演示代码,修改 CourseBuilder 类,将 Course 变为 CourseBuilder 的内部类,将构建步骤添加进去,每完成一个步骤都返回 this
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 build() {
return this.course;
}
@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 + '\'' +
'}';
}
}
}
客户端代码
public class Test {
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());
}
}
这样写法是不是很眼熟,好像在哪见过呢?后面我们分析建造者模式在源码中的应用大家就会明白.接下来,我们再来看一下类图的变化∶
image
4. 建造者模式应用案例
下面我们再来看一个实战案例,这个案例参考了开源框架 JPA 的 SQL 构造模式.是否记得我们在构造 SQL 查询条件的时候,需要根据不同的条件来拼接 SQL 字符串.如果查询条件复杂的时候,我们 SQL 拼接的过程也会变得非常复杂,从而给我们的代码维护带来非常大的困难.因此,我们用建造者类 QueryRuleSqlBuilder 将复杂的构造 SQL 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.来看代码,先创建 QueryRule 类
/**
* QueryRule,主要功能用于构造查询条件
*/
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;
}
public QueryRule orNotIn(String propertyName, Object... values) {
this.ruleList.add(new Rule(NOTIN, propertyName, values).setAndOr(OR));
return this;
}
public QueryRule andNotEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(NOTEQ, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule andGreaterThan(String propertyName, Object value) {
this.ruleList.add(new Rule(GT, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule andGreaterEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(GE, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule andLessThan(String propertyName, Object value) {
this.ruleList.add(new Rule(LT, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule andLessEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(LE, propertyName, new Object[] { value }).setAndOr(AND));
return this;
}
public QueryRule orIsNull(String propertyName) {
this.ruleList.add(new Rule(ISNULL, propertyName).setAndOr(OR));
return this;
}
public QueryRule orIsNotNull(String propertyName) {
this.ruleList.add(new Rule(ISNOTNULL, propertyName).setAndOr(OR));
return this;
}
public QueryRule orIsEmpty(String propertyName) {
this.ruleList.add(new Rule(ISEMPTY, propertyName).setAndOr(OR));
return this;
}
public QueryRule orIsNotEmpty(String propertyName) {
this.ruleList.add(new Rule(ISNOTEMPTY, propertyName).setAndOr(OR));
return this;
}
public QueryRule orLike(String propertyName, Object value) {
this.ruleList.add(new Rule(LIKE, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
public QueryRule orEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(EQ, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
public QueryRule orBetween(String propertyName, Object... values) {
this.ruleList.add(new Rule(BETWEEN, propertyName, values).setAndOr(OR));
return this;
}
public QueryRule orIn(String propertyName, List<Object> values) {
this.ruleList.add(new Rule(IN, propertyName, new Object[] { values }).setAndOr(OR));
return this;
}
public QueryRule orIn(String propertyName, Object... values) {
this.ruleList.add(new Rule(IN, propertyName, values).setAndOr(OR));
return this;
}
public QueryRule orNotEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(NOTEQ, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
public QueryRule orGreaterThan(String propertyName, Object value) {
this.ruleList.add(new Rule(GT, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
public QueryRule orGreaterEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(GE, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
public QueryRule orLessThan(String propertyName, Object value) {
this.ruleList.add(new Rule(LT, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
public QueryRule orLessEqual(String propertyName, Object value) {
this.ruleList.add(new Rule(LE, propertyName, new Object[] { value }).setAndOr(OR));
return this;
}
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
/**
* 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>();
/**
* 或得查询条件
* @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;
}
/**
* 获取参数列表
* @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],"");
}
/**
* 处理 =
* @param rule
*/
private void processEqual(QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),"=",rule.getValues()[0]);
}
/**
* 处理 <>
* @param rule
*/
private void processNotEqual(QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),"<>",rule.getValues()[0]);
}
/**
* 处理 >
* @param rule
*/
private void processGreaterThen(
QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),">",rule.getValues()[0]);
}
/**
* 处理>=
* @param rule
*/
private void processGreaterEqual(
QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),">=",rule.getValues()[0]);
}
/**
* 处理<
* @param rule
*/
private void processLessThen(QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),"<",rule.getValues()[0]);
}
/**
* 处理<=
* @param rule
*/
private void processLessEqual(
QueryRule.Rule rule) {
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
add(rule.getAndOr(),rule.getPropertyName(),"<=",rule.getValues()[0]);
}
/**
* 处理 is null
* @param rule
*/
private void processIsNull(QueryRule.Rule rule) {
add(rule.getAndOr(),rule.getPropertyName(),"is null",null);
}
/**
* 处理 is not null
* @param rule
*/
private void processIsNotNull(QueryRule.Rule rule) {
add(rule.getAndOr(),rule.getPropertyName(),"is not null",null);
}
/**
* 处理 <>''
* @param rule
*/
private void processIsNotEmpty(QueryRule.Rule rule) {
add(rule.getAndOr(),rule.getPropertyName(),"<>","''");
}
/**
* 处理 =''
* @param rule
*/
private void processIsEmpty(QueryRule.Rule rule) {
add(rule.getAndOr(),rule.getPropertyName(),"=","''");
}
/**
* 处理 in 和 not in
* @param rule
* @param name
*/
private void inAndNotIn(QueryRule.Rule rule,String name){
if (ArrayUtils.isEmpty(rule.getValues())) {
return;
}
if ((rule.getValues().length == 1) && (rule.getValues()[0] != null)
&& (rule.getValues()[0] instanceof List)) {
List<Object> list = (List) rule.getValues()[0];
if ((list != null) && (list.size() > 0)){
for (int i = 0; i < list.size(); i++) {
if(i == 0 && i == list.size() - 1){
add(rule.getAndOr(),rule.getPropertyName(),"",name + " (",list.get(i),")");
}else if(i == 0 && i < list.size() - 1){
add(rule.getAndOr(),rule.getPropertyName(),"",name + " (",list.get(i),"");
}
if(i > 0 && i < list.size() - 1){
add(0,"",",","",list.get(i),"");
}
if(i == list.size() - 1 && i != 0){
add(0,"",",","",list.get(i),")");
}
}
}
} else {
Object[] list = rule.getValues();
for (int i = 0; i < list.length; i++) {
if(i == 0 && i == list.length - 1){
add(rule.getAndOr(),rule.getPropertyName(),"",name + " (",list[i],")");
}else if(i == 0 && i < list.length - 1){
add(rule.getAndOr(),rule.getPropertyName(),"",name + " (",list[i],"");
}
if(i > 0 && i < list.length - 1){
add(0,"",",","",list[i],"");
}
if(i == list.length - 1 && i != 0){
add(0,"",",","",list[i],")");
}
}
}
}
/**
* 处理 not in
* @param rule
*/
private void processNotIN(QueryRule.Rule rule){
inAndNotIn(rule,"not in");
}
/**
* 处理 in
* @param rule
*/
private void processIN(QueryRule.Rule rule) {
inAndNotIn(rule,"in");
}
/**
* 处理 order by
* @param rule 查询规则
*/
private void processOrder(Rule rule) {
switch (rule.getType()) {
case QueryRule.ASC_ORDER:
// propertyName 非空
if (!StringUtils.isEmpty(rule.getPropertyName())) {
orders.add(Order.asc(rule.getPropertyName()));
}
break;
case QueryRule.DESC_ORDER:
// propertyName 非空
if (!StringUtils.isEmpty(rule.getPropertyName())) {
orders.add(Order.desc(rule.getPropertyName()));
}
break;
default:
break;
}
}
/**
* 加入到 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,"");
}
/**
* 加入到 sql 查询规则队列
* @param andOr and 或则 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", "") + " ";
}
}
创建 order 类
/**
* 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);
}
}
测试代码
public class Test {
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,我们的客户端代码就非常清朗,来看运行结果
select * from t_member where addr = ? and name like ? order by age asc
5. 建造者模式在源码中的体现
下面来看建造者模式在哪些源码中有应用呢?首先来看 JDK 的 StringBuider,它提供 append() 方法,给我们开放构造步骤,最后调用 toString() 方法就可以获得一个构造好的完整字符串,源码如下∶
public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence
{
...
@Override
public StringBuilder append(String str) {
super.append(str);
return this;
}
...
}
在 MyBatis 中也有体现,比如 CacheBuilder 类
public class CacheBuilder {
...
public Cache build() {
this.setDefaultImplementations();
Cache cache = this.newBaseCacheInstance(this.implementation, this.id);
this.setCacheProperties((Cache)cache);
if (PerpetualCache.class.equals(cache.getClass())) {
Iterator var2 = this.decorators.iterator();
while(var2.hasNext()) {
Class<? extends Cache> decorator = (Class)var2.next();
cache = this.newCacheDecoratorInstance(decorator, (Cache)cache);
this.setCacheProperties((Cache)cache);
}
cache = this.setStandardDecorators((Cache)cache);
} else if (!LoggingCache.class.isAssignableFrom(cache.getClass())) {
cache = new LoggingCache((Cache)cache);
}
return (Cache)cache;
}
...
}
同样在 MyBatis 中,比如 SqlSessionfactoryBuilder 通过调用 build()方法获得的是一个 SqlSessionFactory 类
public class SqlSessionFactoryBuilder {
...
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
SqlSessionFactory var5;
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
var5 = this.build(parser.parse());
} catch (Exception var14) {
throw ExceptionFactory.wrapException("Error building SqlSession.", var14);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException var13) {
}
}
return var5;
}
...
}
当然,在 Spring 中自然也少不了,比如 BeanDefinitionBuilder 通过调用 getBeanDefinition)方法获得一个 BeanDefinition 对象
public class BeanDefinitionBuilder {
...
public AbstractBeanDefinition getBeanDefinition() {
this.beanDefinition.validate();
return this.beanDefinition;
}
...
}
6. 建造者模式的优缺点
6.1 建造者模式的优点
- 封装性好,创建和使用分离
- 扩展性好,建造类之间独立,一定程度上解耦
6.2 建造者模式的缺点
- 产生多余的 Builder 对象
- 产品内部发生变化,建造者都要修改,成本较大
7. 建造者模式和工厂模式的区别
同过前面的学习,我们已经了解建造者模式,那么它和工厂模式有什么区别?
- 建造者模式更加注重方法的调用顺序,工厂模式注重于创建对象
- 创建对象的力度不同,建造者模式创建复杂的对象,由各种复杂的部件组成,工厂模式创建出来的都一样
- 关注重点不一样,工厂模式模式只需要把对象创建出来就可以了,而建造者模式中不仅要创建出这个对象,还要知道这个对象由哪些部件组成
- 建造者模式根据建造过程中的顺序不一样,最终的对象部件组成也不一样
