Fix MVX_USE_SYSTEM_JAVA behavior: force system Java usage without version checking

## Bug Fix

### Problem
When MVX_USE_SYSTEM_JAVA=true, mvx was still checking version compatibility
and falling back to download if versions didn't match exactly. This contradicts
the expected behavior where the flag should mean "always use system Java".

### Solution
- **No version checking**: When MVX_USE_SYSTEM_JAVA=true, use system Java regardless of version
- **No fallback**: Fail with clear error if system Java unavailable (no download fallback)
- **Consistent behavior**: All methods (Install, IsInstalled, GetPath) follow same logic

### Changes
- Install(): Remove version compatibility check, fail if system Java unavailable
- IsInstalled(): Return true if system Java available, false otherwise
- GetPath(): Return system JAVA_HOME or error, no fallback to downloaded version

### Documentation Updates
- Clarify that MVX_USE_SYSTEM_JAVA forces system Java usage
- Document no version checking and no fallback behavior
- Update configuration guide with correct expectations

### Impact
- **Fixes unexpected downloads** when system Java version differs
- **Clearer semantics**: MVX_USE_SYSTEM_JAVA means "always use system"
- **Better CI/CD control**: Predictable behavior in containerized environments
- **Faster builds**: No time wasted on version checking or downloads

This resolves the issue where Java 24 system installation was being rejected
for Java 22 request, causing unnecessary downloads despite the explicit
MVX_USE_SYSTEM_JAVA=true flag.

Author: gnodet

pkg/tools/java.go

@@ -60,27 +60,23 @@ func (j *JavaTool) Install(version string, cfg config.ToolConfig) error {
 
 	// Check if we should use system Java instead of downloading
 	if useSystemJava() {
-		logVerbose("%s=true, attempting to use system Java", getSystemToolEnvVar("java"))
+		logVerbose("%s=true, forcing use of system Java", getSystemToolEnvVar("java"))
 
 		detector := getSystemJavaDetector()
 		systemJavaHome, err := detector.GetSystemHome()
 		if err != nil {
-			logVerbose("System Java not available: %v", err)
-			fmt.Printf("  ⚠️  System Java not available (%v), falling back to download\n", err)
+			return fmt.Errorf("MVX_USE_SYSTEM_JAVA=true but system Java not available: %v", err)
+		}
+
+		systemVersion, err := detector.GetSystemVersion(systemJavaHome)
+		if err != nil {
+			logVerbose("Could not determine system Java version: %v", err)
+			fmt.Printf("  🔗 Using system Java from JAVA_HOME: %s (version detection failed)\n", systemJavaHome)
 		} else {
-			systemVersion, err := detector.GetSystemVersion(systemJavaHome)
-			if err != nil {
-				logVerbose("Could not determine system Java version: %v", err)
-				fmt.Printf("  ⚠️  Could not determine system Java version (%v), falling back to download\n", err)
-			} else if !detector.IsVersionCompatible(systemVersion, version) {
-				logVerbose("System Java version %s does not match requested version %s", systemVersion, version)
-				fmt.Printf("  ⚠️  System Java version %s does not match requested version %s, falling back to download\n", systemVersion, version)
-			} else {
-				// Use system Java by creating a symlink
-				fmt.Printf("  🔗 Using system Java %s from JAVA_HOME: %s\n", systemVersion, systemJavaHome)
-				return detector.CreateSystemLink(systemJavaHome, installDir)
-			}
+			fmt.Printf("  🔗 Using system Java %s from JAVA_HOME: %s\n", systemVersion, systemJavaHome)
 		}
+
+		return detector.CreateSystemLink(systemJavaHome, installDir)
 	}
 
 	// Create installation directory
@@ -110,18 +106,16 @@ func (j *JavaTool) IsInstalled(version string, cfg config.ToolConfig) bool {
 		distribution = "temurin"
 	}
 
-	// If using system Java, check if system Java is available and compatible
+	// If using system Java, check if system Java is available (no version compatibility check)
 	if useSystemJava() {
 		detector := getSystemJavaDetector()
 		if systemJavaHome, err := detector.GetSystemHome(); err == nil {
-			if systemVersion, err := detector.GetSystemVersion(systemJavaHome); err == nil {
-				if detector.IsVersionCompatible(systemVersion, version) {
-					logVerbose("System Java %s is available and compatible with requested version %s", systemVersion, version)
-					return true
-				}
-			}
+			logVerbose("System Java is available at %s (MVX_USE_SYSTEM_JAVA=true)", systemJavaHome)
+			return true
+		} else {
+			logVerbose("System Java not available: %v", err)
+			return false
 		}
-		// If system Java is not available or compatible, fall through to check downloaded version
 	}
 
 	// Try to get the actual Java path (which handles nested directories)
@@ -142,18 +136,15 @@ func (j *JavaTool) GetPath(version string, cfg config.ToolConfig) (string, error
 		distribution = "temurin"
 	}
 
-	// If using system Java, return system JAVA_HOME if available and compatible
+	// If using system Java, return system JAVA_HOME if available (no version compatibility check)
 	if useSystemJava() {
 		detector := getSystemJavaDetector()
 		if systemJavaHome, err := detector.GetSystemHome(); err == nil {
-			if systemVersion, err := detector.GetSystemVersion(systemJavaHome); err == nil {
-				if detector.IsVersionCompatible(systemVersion, version) {
-					logVerbose("Using system Java %s from JAVA_HOME: %s", systemVersion, systemJavaHome)
-					return systemJavaHome, nil
-				}
-			}
+			logVerbose("Using system Java from JAVA_HOME: %s (MVX_USE_SYSTEM_JAVA=true)", systemJavaHome)
+			return systemJavaHome, nil
+		} else {
+			return "", fmt.Errorf("MVX_USE_SYSTEM_JAVA=true but system Java not available: %v", err)
 		}
-		// If system Java is not available or compatible, fall through to check downloaded version
 	}
 
 	installDir := j.manager.GetToolVersionDir("java", version, distribution)

website/content/configuration.md

@@ -312,7 +312,7 @@ export MVX_RETRY_DELAY="5s"
 #### Other System Variables
 
 ```bash
-# Use system-installed tools when available
+# Force use of system-installed tools (no version checking, no fallback)
 export MVX_USE_SYSTEM_JAVA=true
 export MVX_USE_SYSTEM_MAVEN=true
 export MVX_USE_SYSTEM_NODE=true

website/content/tools.md

@@ -97,10 +97,10 @@ export JAVA_HOME=/path/to/your/java
 
 When `MVX_USE_SYSTEM_JAVA=true` is set:
 
-- ✅ **Uses existing Java**: mvx will use the Java installation from `JAVA_HOME`
-- ✅ **Version validation**: Ensures the system Java version matches the requested version
+- ✅ **Always uses system Java**: mvx will use the Java installation from `JAVA_HOME`
+- ✅ **No version checking**: Uses system Java regardless of version differences
 - ✅ **Faster setup**: No time spent downloading Java in CI environments
-- ✅ **Fallback behavior**: If system Java is unavailable or incompatible, falls back to downloading
+- ✅ **Strict behavior**: Fails if system Java is unavailable (no fallback to download)
 
 **Requirements:**
 - `JAVA_HOME` environment variable must be set