java 如何优雅的实现一个mysql where 条件语句来查找数据？

    我们在使用mysql的时候 经常会写where语句 比如 where （age>18 and name="张三"）or age >2 .那么这种是怎么实现的呢？这里我们用一个设计模式 :specification 

     我们首先定义一个接口：

public interface Specification {
	public boolean isSatisfiedBy(T t);
}

isSatisfiedBy() 这个方法是用来检测是否满足我们的条件。后面的所有操作都是基于这个方法来设计，继续定义一个基类：

public abstract class AbstractSpecification implements Specification {
	
	public abstract boolean isSatisfiedBy(T t);
	
	public AbstractSpecification or(Specification s) {
		return new OrSpecification(this,s);
	}

	public AbstractSpecification and(Specification s) {
		return new AndSpecification(this,s);
	}

	public AbstractSpecification not() {
		return new NotSpecification(this);
	}

}

 这个基类里实现了specification 接口 提供了一个抽象方法 isSatisfiedBy()给子类覆盖

添加三个子类 and or not 

public class AndSpecification extends AbstractSpecification {

	private Set> set = new HashSet>();
	
	public AndSpecification(Specification a, Specification b) {
		set.add(a);
		set.add(b);
	}
	
	public boolean isSatisfiedBy(T t) {
		for( Specification s : set ) {
			if( !s.isSatisfiedBy(t) ) {
				return false;
			}
		}
		return true;
	}
	
	@Override
	public AbstractSpecification and(Specification s) {
		set.add(s);
		return this;
	}

}



public class NotSpecification extends AbstractSpecification {

	private Specification spec;
	
	public NotSpecification(Specification s) {
		this.spec=s;
	}
	
	@Override
	public boolean isSatisfiedBy(T t) {
		return !spec.isSatisfiedBy(t);
	}

}


public class OrSpecification extends AbstractSpecification {

	private Set> set = new HashSet>();
	
	public OrSpecification(Specification a, Specification b) {
		set.add(a);
		set.add(b);
	}
	
	public boolean isSatisfiedBy(T t) {
		for( Specification s : set ) {
			if( s.isSatisfiedBy(t) ) {
				return true;
			}
		}
		return false;
	}
	
	@Override
	public AbstractSpecification or(Specification s) {
		set.add(s);
		return this;
	}

}

 基本的框架已经达成了 ，现在实现一个 年龄大于num的类：

public class AgeMoreThanSpecification extends AbstractSpecification {
	
	private int age;
	 
	
	public AgeMoreThanSpecification (int age) {
		super();
		this.age= age;
	 
	}
	
	public boolean isSatisfiedBy(People t) {
		return t.age>age;
	}

}

继续加一个name 等于 xxx的类：

public class NameSpecification extends AbstractSpecification {

	private String name;

	public ToyTypeNameSpecification(String name) {
		super();
		this.name = name;
	}

	@Override
	public boolean isSatisfiedBy(People t) {
		return t.getName().equals(name);
	}

}

ok  返回看开始说的那个sql 条件 where （age>18 and name="张三"）or age >2

我可以构造一个people list 来找这些满足的条件

//此处为伪代码
People p=new People("张三",19);
People p2=new People("张三1",1);
People p3=new People("张三2",1);
People p4=new People("张三3",14);

List.add(p);

List.add(p2);

List.add(p3);

List.add(p4);


Specification specification=new OrSpecification(new AndSpecification(new AgeMoreThanSpecification(18),new NameSpecification("张三")),new AgeMoreThanSpecification(2));

for l:list
if(specification.isSatisfiedBy(l)){

 //find 2 item  People p4=new People("张三3",14);  People p=new People("张三",19); 
}

 优点代码的实现全部隐藏,只暴露过程。 缺点就是类的数量比较多。添加一个条件就是一个子类。