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Maven compiler plugin Unsupported class file major version 60

I am updating a Spigot (Minecraft) plugin and the newest version of Spigot requires Java 16. In my pom I changed the maven compiler plugin target to 16 and the source is still 1.8. Now I am getting the following errors:
Failed to execute goal org.apache.maven.plugins:maven-shade-plugin:3.2.4:shade (default) on project Plugin: Error creating shaded jar: Problem shading JAR C:\Users\Trent\workspace\Stocks\Plugin\target\Plugin-1.0-SNAPSHOT.jar entry com/tchristofferson/stocks/commands/StockbrokerCommand.class: java.lang.IllegalArgumentException: Unsupported class file major version 60
pom:
<?xml version="1.0" encoding="UTF-8"?>
4.0.0
<groupId>com.tchristofferson</groupId>
<artifactId>Stocks</artifactId>
<packaging>pom</packaging>
<version>1.0-SNAPSHOT</version>
<modules>
<module>API</module>
<module>Plugin</module>
</modules>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.1</version>
<configuration>
<source>1.8</source>
<target>16</target>
<release>16</release>
</configuration>
</plugin>
</plugins>
</build>
<distributionManagement>
<snapshotRepository>
<id>ossrh</id>
<url>https://oss.sonatype.org/content/repositories/snapshots</url>
</snapshotRepository>
<repository>
<id>ossrh</id>
<url>https://oss.sonatype.org/service/local/staging/deploy/maven2/</url>
</repository>
</distributionManagement>

I had to use this before I could use 3.3.0-SNAPSHOT
<pluginRepositories>
<pluginRepository>
<id>maven-snapshots</id>
<url>https://repository.apache.org/content/repositories/snapshots/</url>
</pluginRepository>
</pluginRepositories>

#wemu was correct that the maven shade plugin doesn't yet support Java 16. To solve the issue I had to use a snapshot version of the maven shade plugin (3.3.0-SNAPSHOT) since 3.2.4 doesn't support Java 16 yet.

To elaborate on the answer #tchristofferson has given, I got it working by setting snapshots to true in my pluginRepository:
<pluginRepositories>
<pluginRepository>
<releases>
<updatePolicy>never</updatePolicy>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
<id>central</id>
<name>Maven Plugin Repository</name>
<url>http://repo1.maven.org/maven2</url>
</pluginRepository>
</pluginRepositories>
If you don't have the above in your pom.xml, just add it somewhere within <project></project>.
And then change the version of the maven-shade-plugin to this:
<version>3.3.0-SNAPSHOT</version>

In my case, the latest version of maven was installed on my machine, and code was intended for the java 11 version. So I have used an older version of maven and the error didn't appear.

FWIW...
You could also override the version of ASM, in order to make this work with maven-shade-plugin 3.2.x, like so:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>3.2.4</version>
<dependencies>
<dependency>
<groupId>org.ow2.asm</groupId>
<artifactId>asm</artifactId>
<version>9.2</version>
</dependency>
<dependency>
<groupId>org.ow2.asm</groupId>
<artifactId>asm-commons</artifactId>
<version>9.2</version>
</dependency>
</dependencies>
...
</plugin>
References:
https://issues.apache.org/jira/browse/MSHADE-407

How to download maven dependency as *.jar file?

I attempt to implement the example from here, but upon the Maven dependencies installation I can't find jar file amongst downloaded dependencies.
My pom.xml looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<!-- This is often your domain name (reversed.) -->
<groupId>com.abc</groupId>
<!-- The name of this project (actually, the name of the artifact, which is the thing that this project produces. A jar in this case.) -->
<artifactId>javaparser-maven-sample</artifactId>
<!-- The version of this project. SNAPSHOT means "we're still working on it" -->
<version>1.0-SNAPSHOT</version>
<properties>
<!-- Tell Maven we want to use Java 8 -->
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
<!-- Tell Maven to treat all source files as UTF-8 -->
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<geotools.version>2.5.7</geotools.version>
</properties>
<repositories>
<repository>
<id>maven2-repository.dev.java.net</id>
<name>Java.net repository</name>
<url>http://download.java.net/maven/2</url>
</repository>
<repository>
<id>osgeo</id>
<name>Open Source Geospatial Foundation Repository</name>
<url>http://download.osgeo.org/webdav/geotools/</url>
</repository>
<repository> <!--Add the snapshot repository here-->
<snapshots>
<enabled>true</enabled>
</snapshots>
<id>opengeo</id>
<name>OpenGeo Maven Repository</name>
<url>http://repo.opengeo.org</url>
</repository>
</repositories>
<dependencies>
<!-- Here are all your dependencies. Currently only one. These are automatically downloaded from https://mvnrepository.com/ -->
<dependency>
<groupId>com.github.javaparser</groupId>
<artifactId>javaparser-core</artifactId>
<version>3.15.21</version>
</dependency>
<!-- https://mvnrepository.com/artifact/mil.nga.geopackage/geopackage -->
<dependency>
<groupId>mil.nga.geopackage</groupId>
<artifactId>geopackage</artifactId>
<version>3.5.0</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.geotools/gt-api -->
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-api</artifactId>
<version>20.5</version>
</dependency>
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-epsg-hsql</artifactId>
<version>${geotools.version}</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.geotools/gt-geometry -->
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-geometry</artifactId>
<version>2.5-M2</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.geotools/gt-referencing -->
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-referencing</artifactId>
<version>17.1</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.locationtech.jts.io/jts-io-common -->
<dependency>
<groupId>org.locationtech.jts.io</groupId>
<artifactId>jts-io-common</artifactId>
<version>1.16.1</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.opengis/geoapi -->
<dependency>
<groupId>org.opengis</groupId>
<artifactId>geoapi</artifactId>
<version>3.0.0</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.geotools/gt-jts-wrapper -->
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-jts-wrapper</artifactId>
<version>17.0</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-swing</artifactId>
<version>17.0</version>
</dependency>
</dependencies>
<!-- This blob of configuration tells Maven to make the jar executable. You can run it with:
mvn clean package
java -jar target/javaparser-maven-sample-1.0-SNAPSHOT-shaded.jar
-->
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>3.2.3</version>
<executions>
<execution>
<goals>
<goal>shade</goal>
</goals>
<configuration>
<shadedArtifactAttached>true</shadedArtifactAttached>
<transformers>
<transformer
implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
<mainClass>com.abc.conversion.LogicPositivizer</mainClass>
</transformer>
</transformers>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
I am able to download all the jars except geotools related jars. When I clean and run the project, in the .m2 folder I am not able to see the geotool related jars. Even in the maven repository I am not able to download the jar file.
Maven repo
Is there any alternative way to proceed?

The repository run by Boundless has been replaced by one hosted by OSGeo. The OSGeo webdav repo was merged into the new OSGeo repo. Details are here (https://docs.geotools.org/latest/userguide/tutorial/quickstart/maven.html)
Replace this block
<repository>
<id>osgeo</id>
<name>Open Source Geospatial Foundation Repository</name>
<url>http://download.osgeo.org/webdav/geotools/</url>
</repository>
<repository> <!--Add the snapshot repository here-->
<snapshots>
<enabled>true</enabled>
</snapshots>
<id>opengeo</id>
<name>OpenGeo Maven Repository</name>
<url>http://repo.opengeo.org</url>
</repository>
with
<repository>
<id>osgeo</id>
<name>OSGeo Release Repository</name>
<url>https://repo.osgeo.org/repository/release/</url>
<snapshots><enabled>false</enabled></snapshots>
<releases><enabled>true</enabled></releases>
</repository>
<repository>
<id>osgeo-snapshot</id>
<name>OSGeo Snapshot Repository</name>
<url>https://repo.osgeo.org/repository/snapshot/</url>
<snapshots><enabled>true</enabled></snapshots>
<releases><enabled>false</enabled></releases>
</repository>

If you paste the Open Source Geospatial Foundation Repository URL in your web browser and hit enter it will return 404 Not Found error. This happens when Maven attempts to connect to that resource to fetch dependency for you but it's no longer available. However, if you pay close attention to Maven Repo link, there is a note:
Note: this artefact is located at Boundless repository (https://repo.boundlessgeo.com/main/)
Try to replace http://download.osgeo.org/webdav/geotools/ with a given in the note URL and run mvn clean install
Please let me know if that worked for you.

Maven can't resolve the Kotlin Maven Plugin jar

Not sure how to fix this.
I want this version of the Kotlin runtime and maven plugin.
These are the bits in my pom.xml:
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-runtime</artifactId>
<version>1.2-M2</version>
</dependency>
<build>
<plugins>
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
<version>1.2-M2</version>
<executions>
And I added this as a repo:
<repository>
<snapshots>
<enabled>false</enabled>
</snapshots>
<id>kotlin-bintray</id>
<name>Kotlin Bintray</name>
<url>http://dl.bintray.com/kotlin/kotlin-dev/</url>
</repository>
I get this error:
Failure to find
org.jetbrains.kotlin:kotlin-maven-plugin:jar:1.2-M2 in
https://repo.maven.apache.org/maven2 was cached in the local
repository, resolution will not be reattempted until the update
interval of central has elapsed or updates are
forced
But I don't see anything that might be wrong.
By the way, notice that the runtime jar is found, so the repository section must be correct since this repository is where maven finds it. The maven plugin jar is a different matter for some reason though...

I just fixed. It was something really silly. I found out that for plugins one needs to define a plugin repository section.
<pluginRepositories>
<pluginRepository>
<id>kotlin-bintray</id>
<name>Kotlin Bintray</name>
<url>http://dl.bintray.com/kotlin/kotlin-dev</url>
<releases>
<enabled>true</enabled>
</releases>
<snapshots>
<enabled>false</enabled>
</snapshots>
</pluginRepository>
</pluginRepositories>
And now it works. I guess I should spend more time learning maven in depth :)

To make sure it downloads fresh from maven central you will need to blow away your local copy, so delete the directory:
~/.m2/repo/org/jetbrains/kotlin/kotlin-maven-plugin
You will also need to add the 3rd party repo to your settings.xml at ~/.m2 see here
<settings>
...
<profiles>
...
<profile>
<id>myprofile</id>
<repositories>
<repository>
<id>my-repo2</id>
<name>your custom repo</name>
<url>https://dl.bintray.com/kotlin/kotlin-dev/</url>
</repository>
</repositories>
</profile>
...
</profiles>
<activeProfiles>
<activeProfile>myprofile</activeProfile>
</activeProfiles>
...
</settings>

At the documentation, we can see how to implement Compiler Plugins
For kotlin-allopen add the followin <configuration> and <pluginOptions> inside the <plugin> tag
<plugin>
<artifactId>kotlin-maven-plugin</artifactId>
<groupId>org.jetbrains.kotlin</groupId>
<version>${kotlin.version}</version>
<configuration>
<compilerPlugins>
<!-- Or "spring" for the Spring support -->
<plugin>all-open</plugin>
</compilerPlugins>
<pluginOptions>
<!-- Each annotation is placed on its own line -->
<option>all-open:annotation=com.my.Annotation</option>
<option>all-open:annotation=com.their.AnotherAnnotation</option>
</pluginOptions>
</configuration>
<dependencies>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-allopen</artifactId>
<version>${kotlin.version}</version>
</dependency>
</dependencies>
</plugin>

i just added this line and it worked for me
<version>${kotlin.version}</version>

Return code is: 409, ReasonPhrase:Conflict (JCenter)

I have a artificact deployed in JCenter (oss.jfrog.org) although the deployment did not end without error (see Deploy SNAPSHOT to oss.jfrog.org (JCenter)), the jars are there when I check the Repository browser.
Now I add the dependency in a project for this artifact (library) and adding:
<repositories>
<!-- Release repository -->
<repository>
<id>oss-jfrog-artifactory-releases</id>
<name>oss-jfrog-artifactory-releases</name>
<url>http://oss.jfrog.org/artifactory/oss-release-local</url>
</repository>
<!-- Snapshot repository -->
<repository>
<id>oss-jfrog-artifactory-snapshots</id>
<name>oss-jfrog-artifactory-snapshots</name>
<url>http://oss.jfrog.org/artifactory/oss-snapshot-local</url>
</repository>
</repositories>
When maven started building, it throws this error:
Failed to transfer file: http://oss.jf
rog.org/artifactory/oss-release-local/com/myorg/mylibrary/0.0.1-SNAPSHOT/mylibrary-0.0.1-SNAPSHOT.pom. Return code is: 409, ReasonPhrase:Conflict. -> [Help 1]
for the dependency I added. What could be the problem here?

Try using the virtual repositories
<repositories>
<!-- Release repository -->
<repository>
<id>oss-jfrog-artifactory-releases</id>
<name>oss-jfrog-artifactory-releases</name>
<url>http://oss.jfrog.org/artifactory/libs-release</url>
</repository>
<!-- Snapshot repository -->
<repository>
<id>oss-jfrog-artifactory-snapshots</id>
<name>oss-jfrog-artifactory-snapshots</name>
<url>http://oss.jfrog.org/artifactory/libs-snapshot</url>
</repository>
</repositories>

I have a workaround. No idea why, but in my case adding shade plugin to all modules solved the problem, even an empty one:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<configuration>
<artifactSet>
</artifactSet>
<relocations>
</relocations>
</configuration>
</plugin>
</plugins>
</build>

Cannot build Spring 4 project with Maven

I am going through this guide:
https://spring.io/guides/gs/rest-service/
I use Maven for building, so I've fetched the pom.xml linked in the official Spring guide:
https://github.com/spring-guides/gs-rest-service/blob/master/initial/pom.xml
<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.springframework</groupId>
<artifactId>gs-rest-service</artifactId>
<version>0.1.0</version>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.0.1.RELEASE</version>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
</dependency>
</dependencies>
<properties>
<start-class>hello.Application</start-class>
</properties>
<build>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
</plugin>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-build</artifactId>
</plugin>
</plugins>
</build>
<repositories>
<repository>
<id>spring-snapshots</id>
<url>http://repo.spring.io/libs-snapshot</url>
<snapshots><enabled>true</enabled></snapshots>
</repository>
<repository>
<id>spring-repo</id>
<name>Spring Repository</name>
<url>http://repo.spring.io/release</url>
</repository>
</repositories>
<pluginRepositories>
<pluginRepository>
<id>spring-snapshots</id>
<url>http://repo.spring.io/libs-snapshot</url>
<snapshots><enabled>true</enabled></snapshots>
</pluginRepository>
</pluginRepositories>
</project>
I get the following error when running mvn install
[ERROR] Error resolving version for plugin
org.springframework.boot:spring-boot-maven-build' from the
repositories [local (C:\Users\Laszlo_Szucs.m2\repository),
spring-snapshots (repo.spring.io/libs-snapshot), central
repo.maven.apache.org/maven2)]: Plugin not found in any plugin
repository -> [Help 1]
How do I know which version to provide in the pom.xml for this?

Check whether http://repo.spring.io/libs-snapshot is in pom.xml. If not, Add http://repo.spring.io/libs-snapshot to maven repository.
<repository>
<id>spring-repo</id>
<name>Spring Repository</name>
<url>http://repo.spring.io/release</url>
</repository>
And upgrade the maven to 3.0.5 above.

In my case, removing the ~/.m2/repository/org/springframework/boot folder and then cleaning the project resolved the issue. After this issue, I also faced another issue in which STS complained that my maven project was not updated. However, right clicking on the issues in the Markers area and selecting Quick Fix popped up a window which prompted me to update the maven projects. Selecting the projects and clicking the update button in this window resolved the issue.

I specified d goals as "spring-boot:run" (without quote) it works for me
procedure: right click on myproject>run as >run configurations >click on mavenbuild>goals(specify goal name)>run