用Maven编译和执行JDK预览功能

Question

在JDK/12 EarlyAccess Build10中，JEP-325开关表达式已经作为预览特性集成在JDK中。表达式的示例代码（在JEP中也是如此）：

Scanner scanner = new Scanner(System.in);
Day day = Day.valueOf(scanner.next());
switch (day) {
    case MONDAY, TUESDAY -> System.out.println("Back to work.") ;
    case WEDNESDAY -> System.out.println("Wait for the end of week...") ;
    case THURSDAY,FRIDAY -> System.out.println("Plan for the weekend?");
    case SATURDAY, SUNDAY -> System.out.println("Enjoy the holiday!");
}

其中day作为枚举

public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

Answer 1

可以使用以下maven配置，使用--enable-preview以及--release12+（例如，13、14、15)参数来编译代码。

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.8.0</version>
            <configuration>
                <release>12</release> <!-- <release>13/14/15</release> -->
                <compilerArgs>--enable-preview</compilerArgs>
            </configuration>
        </plugin>
        <!-- This is just to make sure the class is set as main class to execute from the jar-->
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-shade-plugin</artifactId>
            <version>3.1.0</version>
            <executions>
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>shade</goal>
                    </goals>
                    <configuration>
                        <transformers>
                            <transformer
                                    implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
                            <transformer
                                    implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                                <mainClass>edu.forty.bits.expression.SwitchExpressions</mainClass>
                            </transformer>
                        </transformers>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

注意：-我还必须在MacOS上确保我的~/.mavenrc文件被配置为将Java13标记为Maven配置的默认java。

执行maven命令从模块类构建jar

mvn clean verify 

java --enable-preview -jar target/forty-bits-of-java-1.0.0-SNAPSHOT.jar

最后一个参数是到Maven构建的jar的路径。

这将产生预期的输出：

（截图来自之前的一次执行。）

GitHub上的来源

<compilerArgs>
    <arg>--enable-preview</arg>
</compilerArgs>

原因是,如果您指定了两个不同的参数,它在配置验证期间不会失败,并且稍后找到的参数会推翻有效配置：

<compilerArgs>--enable-preview</compilerArgs>
<compilerArgs>-Xlint:all</compilerArgs>