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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。

@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 + '\'' +
                '}';
    }
}

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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。

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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。

    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。

2 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。

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@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。

    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。

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。

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。

/**
 * 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。

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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