问题描述
在
(屏幕截图来自之前的执行.)
GitHub 上的源代码
编辑:从不需要的调试会话中学习,使用格式如下的参数:
<compilerArgs><arg>--启用预览</arg></compilerArgs>
原因是,如果您指定两个不同的参数,它在配置验证期间不会失败,并且稍后找到的参数会否决有效配置:
<compilerArgs>--enable-preview</compilerArgs><compilerArgs>-Xlint:all</compilerArgs>
With JDK/12 EarlyAccess Build 10, the JEP-325 Switch Expressions has been integrated as a preview feature in the JDK. A sample code for the expressions (as in the JEP as well):
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!"); }
where Day being an enum as
public enum Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }
The Preview Language and VM Features JEP-12 already elaborate how a feature can be enabled during compile and runtime using javac and java.
How can one try out this feature using Maven?
Step 1
One can make use of the following maven configurations to compile the code using the --enable-preview along with --release 12+ (e.g. 13, 14, 15) argument.
<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>
Note:- I had to also ensure on my MacOS that my ~/.mavenrc file was configured to mark java 13 as the default java configured for maven.
Step 2
Execute the maven command to build the jar from the module classes
mvn clean verify
Step 3
Use the command line to execute the main class of the jar created in the previous step as :
java --enable-preview -jar target/forty-bits-of-java-1.0.0-SNAPSHOT.jar
the last argument is the path to the jar built by maven.
This produces the output as expected as:
(screenshot is from a previous execution.)
Source on GitHub
Edit: A learning from an unwanted debugging session, use the arguments in the format as follows:
<compilerArgs> <arg>--enable-preview</arg> </compilerArgs>
Reason being, if you specify two different arguments it doesn't fail during the configuration validation and the one found later overrules the effective config:
<compilerArgs>--enable-preview</compilerArgs> <compilerArgs>-Xlint:all</compilerArgs>