SpringBoot实践：Java8流处理stream与completableFuture异步编程
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目录
流处理stream结合函数式编程规范，极大提升编程效率，减少代码量，而异步编程completableFuture的使用，则能够更加清晰地表述出业务逻辑，方法的异步调用关系路径很好滴描述了业务脉络；
Stream
这里先构造2个对象，student和teacher，teacher拥有多个student对象，经常有类似使用teacher对象后对student操作的场景，使用流处理比for、foreach、Iterator更优雅；
package org.example;
import org.example.model.Student;
import org.example.model.Teacher;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;
/**
* Hello world!
*
*/
public class App {
public static void main(String[] args) {
System.out.println("begin.");
Student a = new Student(1, "001", "zhangsan", "1");
Student b = new Student(2, "001", "lisi", "2");
Student c = new Student(3, "001", "wangwu", "3");
Student d = new Student(4, "001", "mazi", "4");
Student e = new Student(5, "001", "zhaowu", "5");
Student f = new Student(6, "001", "xuliu", "6");
Student g = new Student(7, "001", "dongqi", "7");
Student h = new Student(8, "001", "chenba", "8");
List<Student> studentList = new ArrayList<Student>();
studentList.add(a);
studentList.add(b);
studentList.add(c);
studentList.add(d);
studentList.add(e);
studentList.add(f);
studentList.add(g);
studentList.add(h);
Teacher t1 = new Teacher(1, "12", "boy", "laoshi", studentList);
}
}
map
map通过映射直接拿到属性值，返回的是属性而并非原有对象，有多重写法：
List<Integer> list = t1.getStudentList()
.stream()
.map(Student::getId)
.collect(Collectors.toList());
List<Integer> list1 = t1.getStudentList()
.stream()
.map(student -> student.getId())
.collect(Collectors.toList());
System.out.println(list);
System.out.println(list1);也可以在取值时候做转换运算：
// map做运算
List<Integer> list2 = t1.getStudentList()
.stream()
.map(student -> {
return student.getId() + 1;
}).collect(Collectors.toList());
System.out.println("list2：" + list2);filter
filter过滤，返回的是原有对象：
// filter过滤
List<Student> list3 = t1.getStudentList()
.stream()
.filter(student -> {
return student.getId() > 3;
}).filter(student -> {
return student.getName().contains("s");
}).collect(Collectors.toList());
System.out.println("list3:" + list3);
List<Student> list4 = t1.getStudentList()
.stream()
.filter(student -> student.getId() > 2)
.collect(Collectors.toList());
flatmap
flatmap拍扁，跟flink中的流式处理定义相似，输出一对多
// flatmap拍扁,输出值
BinaryOperator<Student> getBiggestID = (x1, x2) -> {
int id1 = x1.getId();
int id2 = x2.getId();
return x1.getId() > x2.getId() ? x1 : x2;
};
Integer maxID = Optional.ofNullable(t1.getStudentList())
.flatMap(students -> students.stream().reduce(getBiggestID))
.map(Student::getId)
.orElse(0);
System.out.println(maxID);collect
可以流处理同样可以聚合，groubpingby函数为例：
// groupby转化
Map<String,List<Student>> list6 = t1.getStudentList().stream()
.filter(student -> !student.getName().isEmpty())
.collect(Collectors.groupingBy(x->x.getName().contains("s")?"ss":"nn"));
System.out.println(list6);CompletableFuture
当需要对Teacher和Student对象进行查询逻辑处理时候，就可以用异步编程相关方法；在方法的定义加上CompletableFuture<>关键词，把返回值放在泛型中，比如CompletableFuture<List<Student>>getStudents()方法，如果不使用异步编程直接返回List，使用异步编程返回CompletableFuture对象，在使用时候；
package org.example.service;
import org.example.model.Student;
import org.example.model.Teacher;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import static java.util.concurrent.CompletableFuture.completedFuture;
public class TeacherServiceImpl {
Student a = new Student(1, "sat", "zhangsan", "1");
Student b = new Student(2, "hhh", "lisi", "2");
Student c = new Student(3, "kg", "wangwu", "3");
Student d = new Student(4, "ss", "mazi", "4");
Student e = new Student(5, "tt", "zhaowu", "5");
Student f = new Student(6, "dfa", "xuliu", "6");
Student g = new Student(7, "safg", "dongqi", "7");
Student h = new Student(8, "tsa", "chenba", "8");
List<Student> students = new ArrayList<Student>();
// 假设需要进行数据库查询操作
public CompletableFuture<List<Student>> getStudents(){
Student a = new Student(1, "sat", "zhangsan", "1");
Student b = new Student(2, "hhh", "lisi", "2");
Student c = new Student(3, "kg", "wangwu", "3");
Student d = new Student(4, "ss", "mazi", "4");
Student e = new Student(5, "tt", "zhaowu", "5");
Student f = new Student(6, "dfa", "xuliu", "6");
Student g = new Student(7, "safg", "dongqi", "7");
Student h = new Student(8, "tsa", "chenba", "8");
List<Student> studentList = new ArrayList<Student>();
studentList.add(a);
studentList.add(b);
studentList.add(c);
studentList.add(d);
studentList.add(e);
studentList.add(f);
studentList.add(g);
studentList.add(h);
studentList.add(h);
return completedFuture(studentList);
}
// 对数据库返回值再加工
public CompletableFuture<List<Teacher>> getTeachers(List<Student> students){
List<Teacher> teachers = new ArrayList<Teacher>();
Teacher t2 = new Teacher(1, "12", "boy", "lixx", students);
Teacher t3 = new Teacher(3, "21", "girl", "wangxx", students);
teachers.add(t2);
teachers.add(t3);
return completedFuture(teachers);
}
}对Student是数据库操作，对teacher是数据库返回的在处理，那么作为impl，就可以组合进行业务逻辑链路：
// 异步编程链路
public static CompletableFuture<String> test(){
return teacherService.getStudents()
.thenApplyAsync(students -> students.stream()
.filter(i->i.getName().contains("s"))
.collect(Collectors.toList()))
.thenApply(students -> teacherService.getTeachers(students).toString())
.toCompletableFuture();
}
关于thencompose和thenapply方法需要搞明白；
supplyAsync(Func)：参数为计算任务（函数），该方法会异步执行传入的Func函数，并且使用get或join方法获取计算结果；
runAsync(Func)：该方法也可以创建CompletableFuture实例，只是适合创建无返回值的Func；
thenApply()：then开头的函数都是函数执行后的操作，thenApply前后两个任务在同一个线程中执行，thenApplyAsyn前后两个任务不是同一个线程，而是两个不同线程，相同点是都要等前一个方法结束后才会执行后面跟着的方法，thenApply函数传入的函数需要有入参和出参；
thenCombine()：与thenApply相比，更强调前后连接任务的关系是并行的，也就是Func可以和后面连接的函数并行处理，最后两个任务均完成将结果同时传递给下游任务。
thenCompose()：当前后两个任务存在关联关系，当第一个任务完成时才会执行第二个操作就可以使用thenCompose方法，两个异步任务全部完成时才会执行某些操作用thenCombine,
whenComplete()：任务完成后的回调；
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