Add multi-release JAR support via classpath attributes and new demo

Author: Copilot

demo/multi-release-jar-classpath/.classpath

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+<?xml version="1.0" encoding="UTF-8"?>
+<classpath>
+	<classpathentry kind="src" path="src"/>
+	<classpathentry kind="src" path="src9">
+		<attributes>
+			<attribute name="release" value="9"/>
+		</attributes>
+	</classpathentry>
+	<classpathentry kind="src" path="src11">
+		<attributes>
+			<attribute name="release" value="11"/>
+		</attributes>
+	</classpathentry>
+	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.8"/>
+	<classpathentry kind="con" path="org.eclipse.pde.core.requiredPlugins"/>
+	<classpathentry kind="output" path="bin"/>
+</classpath>

demo/multi-release-jar-classpath/META-INF/MANIFEST.MF

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+Manifest-Version: 1.0
+Bundle-ManifestVersion: 2
+Bundle-Name: MR-JAR Classpath TestCase
+Bundle-SymbolicName: mr.classpath
+Bundle-Version: 1.0.0.qualifier
+Export-Package: tycho.mr.example;version="0.0.1"
+Import-Package: org.apache.commons.io
+Multi-Release: true
+Require-Capability: osgi.ee;filter:="(&(osgi.ee=JavaSE)(version=1.8))"

demo/multi-release-jar-classpath/README.md

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+# Building Multi-Release-Jar with Classpath Attributes
+
+This sample shows how to build a [Multi-Release-Jar](https://openjdk.org/jeps/238) with Tycho using the JDT classpath attribute approach.
+
+This approach requires the `Multi-Release: true` manifest header but simplifies the build by using Eclipse JDT's `release` classpath attribute to mark source folders for specific Java releases, without requiring special directory structures or supplemental manifests.
+
+## Structure
+
+- `src` - contains the main sources (Java 8)
+- `src9` - contains the source for release 9 (marked with `release="9"` in `.classpath`)
+- `src11` - contains the source for release 11 (marked with `release="11"` in `.classpath`)
+- `META-INF/MANIFEST.MF` - the manifest with `Multi-Release: true` header
+
+Note: Source folders can be named anything (e.g., `src_java9`, `java9-src`), not just `src9` or `src11`.
+
+## Classpath Configuration
+
+The `.classpath` file contains entries like:
+
+```xml
+<classpathentry kind="src" path="src9">
+    <attributes>
+        <attribute name="release" value="9"/>
+    </attributes>
+</classpathentry>
+```
+
+This tells Tycho to compile the sources in `src9` for Java 9 and place them in `META-INF/versions/9/` in the resulting JAR.
+
+## Comparison with Manifest-First Approach
+
+This approach is more flexible than the manifest-first approach because:
+- Source folders can be named flexibly (derived from `.classpath`, not fixed naming convention)
+- No supplemental manifests required in `META-INF/versions/N/OSGI-INF/`
+- Follows Eclipse JDT conventions more closely
+- Easier integration with Eclipse IDE
+
+Both approaches require the `Multi-Release: true` manifest header.
+
+See the `multi-release-jar` demo for the traditional manifest-first approach with fixed directory naming.

demo/multi-release-jar-classpath/build.properties

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+source.. = src/
+output.. = bin/
+bin.includes = META-INF/,\
+               .

demo/multi-release-jar-classpath/pom.xml

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+<?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>tycho.its.compiler.mr.classpath</groupId>
+	<artifactId>mr.classpath</artifactId>
+	<version>1.0.0-SNAPSHOT</version>
+	<packaging>eclipse-plugin</packaging>
+	<properties>
+		<!-- This values are overridden by the test execution but are usefull if running as a standalone project (e.g. debugging) -->
+		<tycho-version>4.0.0-SNAPSHOT</tycho-version>
+		<target-platform>https://download.eclipse.org/releases/2022-12/</target-platform>
+	</properties>
+	<repositories>
+		<repository>
+			<id>platform</id>
+			<url>${target-platform}</url>
+			<layout>p2</layout>
+		</repository>
+	</repositories>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.eclipse.tycho</groupId>
+				<artifactId>tycho-maven-plugin</artifactId>
+				<version>${tycho-version}</version>
+				<extensions>true</extensions>
+			</plugin>
+		</plugins>
+	</build>
+</project>

demo/multi-release-jar-classpath/src/tycho/mr/example/HttpClient.java

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+package tycho.mr.example;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.URL;
+
+import org.apache.commons.io.IOUtils;
+
+public class HttpClient {
+
+	public byte[] fetchBytes(URL url) throws IOException {
+		try (InputStream stream = url.openStream()) {
+			// For Java < 9 we need to use an external library!
+			return IOUtils.toByteArray(stream);
+		}
+	}
+}

demo/multi-release-jar-classpath/src/tycho/mr/example/Main.java

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+package tycho.mr.example;
+
+import java.io.IOException;
+import java.net.URL;
+
+public class Main {
+	public static void main(String[] args) throws IOException {
+		if (args.length == 0) {
+			System.err.println("Please specify at laest one file to fetch!");
+			System.exit(1);
+		}
+		HttpClient client = new HttpClient();
+		for (String arg : args) {
+			byte[] bytes = client.fetchBytes(new URL(arg));
+			System.out.println("URL " + arg + " has provided " + bytes.length + " bytes!");
+		}
+	}
+}

demo/multi-release-jar-classpath/src11/tycho/mr/example/HttpClient.java

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+package tycho.mr.example;
+
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.InterruptedIOException;
+import java.net.HttpURLConnection;
+import java.net.URISyntaxException;
+import java.net.URL;
+import java.net.http.HttpClient.Redirect;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.net.http.HttpResponse.BodyHandlers;
+import java.time.Duration;
+
+public class HttpClient {
+
+	public byte[] fetchBytes(URL url) throws IOException {
+		// For From Java 11 we can even you a true client with HTTP/2 support!
+		java.net.http.HttpClient client = java.net.http.HttpClient.newBuilder()//
+				.followRedirects(Redirect.NORMAL)//
+				.connectTimeout(Duration.ofSeconds(20)) //
+				.build();
+		try {
+			HttpRequest request = HttpRequest.newBuilder().uri(url.toURI()).build();
+			HttpResponse<byte[]> response = client.send(request, BodyHandlers.ofByteArray());
+			if (response.statusCode() == HttpURLConnection.HTTP_NOT_FOUND) {
+				throw new FileNotFoundException(url.toString());
+			}
+			return response.body();
+		} catch (URISyntaxException e) {
+			throw new IOException("invalid: " + url, e);
+		} catch (InterruptedException e) {
+			throw new InterruptedIOException();
+		}
+	}
+}

demo/multi-release-jar-classpath/src9/tycho/mr/example/HttpClient.java

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+package tycho.mr.example;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.URL;
+
+import org.apache.commons.io.IOUtils;
+
+public class HttpClient {
+
+	public byte[] fetchBytes(URL url) throws IOException {
+		try (InputStream stream = url.openStream()) {
+			// For Java >= 9 we can use the build-in
+			return stream.readAllBytes();
+		}
+	}
+}