Java 8 要点

Java 8 要点

1. new Date/Time API
2. Stream
3. Lambda Expressions
4. Method references
5. Optional
6. Default Method

new Date/Time API - java.time

• 老时间库的问题 - why jsr310
  1. java.util.Date和SimpleDateFormat 线程不安全
  2. 月份从1开始，日期从0开始
  3. 两个相同的java.sql.Time实例不equals
  4. java.util.Date vs. java.sql.Timestamp/Date/Time
• 新的时间库的设计思想
  1. Immutable
  2. LocalDate, LocalTime, LocalDateTime
  3. 如果需要时区信息，换另外一套
• 新老时间类型互转
  1. java.sql.Timestamp#toLocalDateTime
  2. java.sql.Timestamp#from
• format

    @Test
    public void format() throws Exception {
        DateTimeFormatter dateFormatter = DateTimeFormatter.ISO_LOCAL_DATE;
        Assert.assertEquals("2011-11-11", dateFormatter.format(LocalDate.of(2011, 11, 11)));
    }

• parse

        @Test
        public void parse() throws Exception {
            DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
            LocalDate day = LocalDate.parse("1900-01-01", dateFormatter);
            Assert.assertEquals(LocalDate.of(1900, 1, 1), day);
        }

• JDBC
  JDBC4.2（mysql java 驱动6.0）开始才支持 Java 8的新的时间库，在使用JDBC4.2之前，使用时需要将新的类型转换成老的类型
• Spring MVC

    @SpringBootApplication
    public class WebMain extends WebMvcConfigurerAdapter {
    
        @Override
        public void addFormatters(FormatterRegistry registry) {
            DateTimeFormatterRegistrar registrar = new DateTimeFormatterRegistrar();
            registrar.setDateFormatter(DateTimeFormatter.ISO_DATE);
            registrar.setTimeFormatter(DateTimeFormatter.ISO_TIME);
            registrar.setDateTimeFormatter(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
            registrar.registerFormatters(registry);
        }
    
    }

• JPA

        @Bean
        public LocalContainerEntityManagerFactoryBean catalogEntityManagerFactory(
                @Qualifier(CatalogRepositoryConfig.UNIT_NAME) DataSource catalogDataSource,
                EntityManagerFactoryBuilder builder, JpaProperties jpaProperties) {
            return builder.dataSource(catalogDataSource)
                    .packages(Server.class, Jsr310JpaConverters.class)
                    .persistenceUnit(UNIT_NAME)
                    .properties(jpaProperties.getHibernateProperties(catalogDataSource))
                    .build();
        }

• Jackson

spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS = false

            
                com.fasterxml.jackson.datatype
                jackson-datatype-jsr310
            

        @Bean
        public SimpleModule jsr310JacksonModule() {
            SimpleModule jsr310Module = new SimpleModule();
            DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
            jsr310Module.addDeserializer(LocalDateTime.class, new LocalDateTimeDeserializer(dateTimeFormatter));
            jsr310Module.addSerializer(LocalDateTime.class, new LocalDateTimeSerializer(dateTimeFormatter));
            return jsr310Module;
        }

• 时间计算

        @Test
        public void plusOrMinus() throws Exception {
            LocalDateTime payTime = LocalDateTime.of(2015, 10, 10, 19, 0);
            Assert.assertEquals(LocalDateTime.of(2015, 10, 30, 19, 0), payTime.plusDays(20));
            Assert.assertEquals(LocalDateTime.of(2015, 10, 11, 5, 0), payTime.plusHours(10));
            Assert.assertEquals(LocalDateTime.of(2015, 9, 30, 19, 0), payTime.minusDays(10));
        }

---

stream

consumer

使用场景

1. groupBy
2. partition
3. sum
4. map
5. filter

注意

1. stream只能consume一次
2. Iterable没有stream
3. stream代表的是一些function的整合，并不是一个数据结构
4. parallel stream 慎用
5. stream的每个function最好是没有副作用的

Lambda Expressions

Method References

---

Optional

1. Optional
2. OptionalInt, OptionalLong, OptionalDouble

参考资料

1. https://leanpub.com/whatsnewinjava8/read
2. https://github.com/adamd/hellojava8
3. https://github.com/katoquro/jdk8-lambda-samples
4. https://octoperf.com/blog/2015/08/24/one-year-with-java-8-in-production/
5. http://zeroturnaround.com/rebellabs/java-parallel-streams-are-bad-for-your-health/
6. http://zeroturnaround.com/rebellabs/how-your-addiction-to-java-8-default-methods-may-make-pandas-sad-and-your-teammates-angry/
7. http://zeroturnaround.com/rebellabs/java-8-explained-default-methods/
8. http://blog.jooq.org/2014/06/13/java-8-friday-10-subtle-mistakes-when-using-the-streams-api/
9. http://blog.jooq.org/2014/04/04/java-8-friday-the-dark-side-of-java-8/
10. http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/
11. http://blog.takipi.com/new-parallelism-apis-in-java-8-behind-the-glitz-and-glamour/
12. http://blog.takipi.com/the-dark-side-of-lambda-expressions-in-java-8/
13. https://robinst.github.io/guava-java8-presentation