详解JDK 12中的新特性

学习笔记 : 详解JDK 12中的新特性

简介

JDK 12 is the open-source reference implementation of version 12 of the Java SE Platform as specified by by JSR 386 in the Java Community Process. JDK 12 reached General Availability on 19 March 2019. Production-ready binaries under the GPL are available from Oracle; binaries from other vendors will follow shortly. The features and schedule of this release were proposed and tracked via the JEP Process, as amended by the JEP 2.0 proposal. The release was produced using the JDK Release Process (JEP 3).

新特性

189: Shenandoah: A Low-Pause-Time Garbage Collector (Experimental)
230: Microbenchmark Suite
325: Switch Expressions (Preview)
334: JVM Constants API
340: One AArch64 Port, Not Two
341: Default CDS Archives
344: Abortable Mixed Collections for G1
346: Promptly Return Unused Committed Memory from G1

时间表

2018/12/13 Rampdown Phase One (fork from main line)
2019/01/17 Rampdown Phase Two
2019/02/07 Release-Candidate Phase
2019/03/19 General Availability

Switch Expressions 特性演示

int score = new Random(100).nextInt();

/*
JDK12之前版本的写法:许多break使它不必要地冗长,如果忘记写break,则会产生意料之外的结果或者异常,所以针对于此jdk12在这里进行了优化升级
*/
switch (score) {
    case 60:
        System.out.println("及格");
        break;
    case 80:
        System.out.println("良好");
        break;
    case 90:
        System.out.println("优秀");
        break;
    default:
        System.out.println("default");
        break;
}

/*
JDK12新特性写法:我们建议引入一种新形式的开关标签,写成"case L ->"表示如果标签匹配,则只执行标签右侧的代码:
*/
switch (score) {
    case 60 -> System.out.println("及格");
    case 80 -> System.out.println("良好");
    case 90 -> System.out.println("优秀");
    default -> System.out.println("default");
}

/*
JDK12前的写法:许多Switch表达式,每个case都会赋值给一个变量或者执行某种操作,如下是赋值给numLetters变量具体值:
*/
int numLetters;
switch (score) {
    case 60:
    case 70:
        numLetters = 6;
        break;
    case 80:
    case 90:
        numLetters = 7;
        break;
    case 100:
        numLetters = 8;
        break;
    default:
        throw new IllegalStateException("score:" + score);
}

/*
JDK12新特性写法:将此表达为一种陈述是迂回的,重复的,并且容易出错. 意味着我们应该计算numLetters每一天的价值,应该可以直接说,使用更清晰,更安全Switch表达式:
*/
int numLetters2 = switch (score) {
    case 60, 70 -> 6;
    case 80, 90 -> 7;
    case 95, 100 -> 8;
    default -> throw new IllegalStateException("count:" + score);
};

/*
如果将它用在方法上,则可以为写成:
*/
static void howMany(int k) {
    switch (k) {
        case 1 -> System.out.println("one");
        case 2 -> System.out.println("two");
        case 3 -> System.out.println("many");
}

/*
或者类上,例如将这个新特性用在抽象工厂类:
*/ 
T result = switch (arg) {
    case L1 -> e1;
    case L2 -> e2;
    default -> e3;
};