Fix #481: Rely on JRE_HOME for Java location

* JRE_HOME is now set in libjvm, see paketo-buildpacks/libjvm#395
* thanks to that, we know for sure that the JRE_HOME/bin/java will be used for the training run

boot/spring_performance.go

@@ -106,7 +106,13 @@ func (s SpringPerformance) Contribute(layer libcnb.Layer) (libcnb.Layer, error)
 			os.RemoveAll(s.AppPath)
 		}
 
-		if err := s.springBootJarCDSLayoutExtract(jarPath); err != nil {
+		javaCommand := "java"
+		jreHome := sherpa.GetEnvWithDefault("JRE_HOME", sherpa.GetEnvWithDefault("JAVA_HOME", ""))
+		if jreHome != "" {
+			javaCommand = jreHome + "/bin/java"
+		}
+
+		if err := s.springBootJarCDSLayoutExtract(javaCommand, jarPath); err != nil {
 			return layer, fmt.Errorf("error extracting Boot jar at %s\n%w", jarPath, err)
 		}
 		startClassValue, _ := s.Manifest.Get("Start-Class")
@@ -140,7 +146,7 @@ func (s SpringPerformance) Contribute(layer libcnb.Layer) (libcnb.Layer, error)
 
 		// perform the training run, application.dsa, the cache file, will be created
 		if err := s.Executor.Execute(effect.Execution{
-			Command: "java",
+			Command: javaCommand,
 			Env:     trainingRunEnvVariables,
 			Args:    trainingRunArgs,
 			Dir:     s.AppPath,
@@ -163,10 +169,10 @@ func (s SpringPerformance) Name() string {
 	return s.LayerContributor.Name
 }
 
-func (s SpringPerformance) springBootJarCDSLayoutExtract(jarPath string) error {
+func (s SpringPerformance) springBootJarCDSLayoutExtract(javaCommand string, jarPath string) error {
 	s.Logger.Bodyf("Extracting Jar")
 	if err := s.Executor.Execute(effect.Execution{
-		Command: "java",
+		Command: javaCommand,
 		Args:    []string{"-Djarmode=tools", "-jar", jarPath, "extract", "--destination", s.AppPath},
 		Dir:     filepath.Dir(jarPath),
 		Stdout:  s.Logger.InfoWriter(),

boot/spring_performance_test.go

@@ -250,8 +250,8 @@ Spring-Boot-Lib: BOOT-INF/lib
 
 		cwd, _ := os.Getwd()
 		old := filepath.Join(cwd, "testdata", "spring-cloud-bindings", "spring-cloud-bindings-1.2.3.jar")
-		new := filepath.Join(ctx.Application.Path, "BOOT-INF", "lib", "spring-cloud-bindings-1.2.3.jar")
-		os.Symlink(old, new)
+		now := filepath.Join(ctx.Application.Path, "BOOT-INF", "lib", "spring-cloud-bindings-1.2.3.jar")
+		os.Symlink(old, now)
 
 		props, err := libjvm.NewManifest(ctx.Application.Path)
 		Expect(err).NotTo(HaveOccurred())
@@ -284,6 +284,40 @@ Spring-Boot-Lib: BOOT-INF/lib
 
 	})
 
+	it("fails with a non existing JRE_HOME path", func() {
+		Expect(os.Setenv("JRE_HOME", "/that/does/not/exist")).To(Succeed())
+
+		aotEnabled, cdsEnabled = true, true
+		dc := libpak.DependencyCache{CachePath: "testdata"}
+		executor.On("Execute", mock.Anything).Return(nil)
+
+		Expect(os.WriteFile(filepath.Join(ctx.Application.Path, "META-INF", "MANIFEST.MF"), []byte(`
+Spring-Boot-Version: 3.3.1
+Spring-Boot-Classes: BOOT-INF/classes
+Spring-Boot-Lib: BOOT-INF/lib
+`), 0644)).To(Succeed())
+		props, err := libjvm.NewManifest(ctx.Application.Path)
+		Expect(err).NotTo(HaveOccurred())
+
+		s := boot.NewSpringPerformance(dc, ctx.Application.Path, props, aotEnabled, cdsEnabled, "", true)
+		s.Executor = executor
+
+		layer, err := ctx.Layers.Layer("test-layer")
+		Expect(err).NotTo(HaveOccurred())
+
+		layer, err = s.Contribute(layer)
+		Expect(err).NotTo(HaveOccurred())
+
+		Expect(executor.Calls).To(HaveLen(2))
+		e, ok := executor.Calls[1].Arguments[0].(effect.Execution)
+		Expect(ok).To(BeTrue())
+
+		Expect(e.Command).To(Equal("/that/does/not/exist/bin/java"))
+		Expect(layer.Build).To(BeTrue())
+
+		Expect(os.Unsetenv("JRE_HOME")).To(Succeed())
+	})
+
 }
 
 func unzip(src, dest string) error {