Select the best matching JRE for an EE from the list of compatible JVMs

org.eclipse.jdt.launching/META-INF/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: %pluginName
 Bundle-SymbolicName: org.eclipse.jdt.launching; singleton:=true
-Bundle-Version: 3.23.500.qualifier
+Bundle-Version: 3.24.0.qualifier
 Bundle-Activator: org.eclipse.jdt.internal.launching.LaunchingPlugin
 Bundle-Vendor: %providerName
 Bundle-Localization: plugin

org.eclipse.jdt.launching/launching/org/eclipse/jdt/internal/launching/JREContainerInitializer.java

@@ -19,7 +19,6 @@
 import java.io.File;
 import java.net.MalformedURLException;
 import java.net.URL;
-import java.util.Arrays;
 
 import org.eclipse.core.runtime.CoreException;
 import org.eclipse.core.runtime.IPath;
@@ -178,31 +177,16 @@ public static IVMInstall resolveVM(IExecutionEnvironment environment) {
 		}
 		IVMInstall vm = environment.getDefaultVM();
 		if (vm == null) {
-			IVMInstall[] installs = environment.getCompatibleVMs();
-			// take the first strictly compatible VM if there is no default
-			if (installs.length == 0 && LaunchingPlugin.DEBUG_JRE_CONTAINER) {
+			vm = environment.getCompatibleVM();
+			if (vm == null) {
 				LaunchingPlugin.trace("\t*** NO COMPATIBLE VMS ***"); //$NON-NLS-1$
+				return null;
 			}
-			for (int i = 0; i < installs.length; i++) {
-				IVMInstall install = installs[i];
-				if (environment.isStrictlyCompatible(install)) {
-					vm = install;
-					if (LaunchingPlugin.DEBUG_JRE_CONTAINER) {
-						LaunchingPlugin.trace("\tPerfect Match: " + vm.getName()); //$NON-NLS-1$
-					}
-					break;
-				}
-			}
-			//try the default VM install: https://bugs.eclipse.org/bugs/show_bug.cgi?id=371300
-			// if default vm is a match https://bugs.eclipse.org/bugs/show_bug.cgi?id=484026
-			if (vm == null && installs.length > 0 && Arrays.asList(installs).contains(JavaRuntime.getDefaultVMInstall())) {
-				vm = JavaRuntime.getDefaultVMInstall();
-			}
-			// use the first VM failing that
-			if (vm == null && installs.length > 0) {
-				vm = installs[0];
-				if (LaunchingPlugin.DEBUG_JRE_CONTAINER) {
-					LaunchingPlugin.trace("\tFirst Match: " + vm.getName()); //$NON-NLS-1$
+			if (LaunchingPlugin.DEBUG_JRE_CONTAINER) {
+				if (environment.isStrictlyCompatible(vm)) {
+					LaunchingPlugin.trace("\tPerfect Match: " + vm.getName()); //$NON-NLS-1$
+				} else {
+					LaunchingPlugin.trace("\tUse compatible VM: " + vm.getName()); //$NON-NLS-1$
 				}
 			}
 		} else {

org.eclipse.jdt.launching/launching/org/eclipse/jdt/internal/launching/environments/ExecutionEnvironment.java

@@ -42,6 +42,7 @@
 import org.eclipse.jdt.core.JavaModelException;
 import org.eclipse.jdt.internal.launching.LaunchingPlugin;
 import org.eclipse.jdt.launching.IVMInstall;
+import org.eclipse.jdt.launching.IVMInstall2;
 import org.eclipse.jdt.launching.IVMInstallChangedListener;
 import org.eclipse.jdt.launching.JavaRuntime;
 import org.eclipse.jdt.launching.LibraryLocation;
@@ -210,6 +211,75 @@ public IVMInstall[] getCompatibleVMs() {
 		return fCompatibleVMs.toArray(new IVMInstall[fCompatibleVMs.size()]);
 	}
 
+	@Override
+	public IVMInstall getCompatibleVM() {
+		init();
+		if (fCompatibleVMs.isEmpty()) {
+			return null;
+		}
+		if (fCompatibleVMs.size() == 1) {
+			return fCompatibleVMs.get(0);
+		}
+		if (!fStrictlyCompatible.isEmpty()) {
+			// first lean to the default if it is strictly compatible
+			IVMInstall workspaceDefaultVMInstall = JavaRuntime.getDefaultVMInstall();
+			if (fStrictlyCompatible.contains(workspaceDefaultVMInstall)) {
+				return workspaceDefaultVMInstall;
+			}
+			return fStrictlyCompatible.iterator().next();
+		}
+		IVMInstall best = null;
+		java.lang.Runtime.Version bestVersion = null;
+		for (IVMInstall vm : fCompatibleVMs) {
+			java.lang.Runtime.Version vmVersion = getRuntimeVersion(vm);
+			if (isBetter(vmVersion, bestVersion)) {
+				bestVersion = vmVersion;
+				best = vm;
+			}
+		}
+		return best;
+	}
+
+	private boolean isBetter(java.lang.Runtime.Version version, java.lang.Runtime.Version other) {
+		if (other == null) {
+			return true;
+		}
+		if (version == null) {
+			return false;
+		}
+		if (version.feature() < other.feature()) {
+			// lower major (== feature) is better!
+			return true;
+		}
+		if (version.feature() > other.feature()) {
+			// higher is not better!
+			return false;
+		}
+		if (version.feature() == 1) {
+			// special case for java 1.x versions, lower minor (== interim) is better!
+			if (version.interim() < other.interim()) {
+				return true;
+			}
+			if (version.interim() > other.interim()) {
+				return false;
+			}
+		}
+		// when we are here we have equally good versions, e.g. two Java 11, we choose now the one that is better in the given interim/patch levels
+		// and can use the default compare of version
+		return version.compareTo(other) > 0;
+	}
+
+	private java.lang.Runtime.Version getRuntimeVersion(IVMInstall vm) {
+		if (vm instanceof IVMInstall2 vm2) {
+			try {
+				return Runtime.Version.parse(vm2.getJavaVersion());
+			} catch (RuntimeException e) {
+				// in this case we can't know the version...
+			}
+		}
+		return null;
+	}
+
 	/* (non-Javadoc)
 	 * @see org.eclipse.jdt.launching.environments.IExecutionEnvironment#isStrictlyCompatible(org.eclipse.jdt.launching.IVMInstall)
 	 */

org.eclipse.jdt.launching/launching/org/eclipse/jdt/launching/environments/IExecutionEnvironment.java

@@ -64,6 +64,19 @@ public interface IExecutionEnvironment {
 	 */
 	public IVMInstall[] getCompatibleVMs();
 
+	/**
+	 * Return the vm that is best matching this environment that is:
+	 *
+	 * <ol>
+	 * <li>if a strictly compatible is found this one is returned, if multiple are strictly compatible it picks one randomly</li>
+	 * <li>in all other case it choose the one with the lowest version, if multiple match the same version it picks one randomly</li>
+	 * </ol>
+	 *
+	 * @return the best compatible VM or <code>null</code> if none is found
+	 * @since 3.24
+	 */
+	public IVMInstall getCompatibleVM();
+
 	/**
 	 * Returns whether the specified VM install is strictly compatible with
 	 * this environment. Returns <code>true</code> to indicate the VM install

org.eclipse.jdt.launching/pom.xml

@@ -18,7 +18,7 @@
   </parent>
   <groupId>org.eclipse.jdt</groupId>
   <artifactId>org.eclipse.jdt.launching</artifactId>
-  <version>3.23.500-SNAPSHOT</version>
+  <version>3.24.0-SNAPSHOT</version>
   <packaging>eclipse-plugin</packaging>
 
   <build>