[issue GoogleContainerTools#4175] SpringBootExplodedProcessor now use…

…s JAR's Main-Class when creating the target image's entrypoint

jib-cli/src/main/java/com/google/cloud/tools/jib/cli/jar/SpringBootExplodedProcessor.java

@@ -35,6 +35,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.function.Predicate;
+import java.util.jar.Attributes;
 import java.util.jar.JarFile;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -135,14 +136,23 @@ public List<FileEntriesLayer> createLayers() throws IOException {
   }
 
   @Override
-  public ImmutableList<String> computeEntrypoint(List<String> jvmFlags) {
-    ImmutableList.Builder<String> entrypoint = ImmutableList.builder();
-    entrypoint.add("java");
-    entrypoint.addAll(jvmFlags);
-    entrypoint.add("-cp");
-    entrypoint.add(JarLayers.APP_ROOT.toString());
-    entrypoint.add("org.springframework.boot.loader.JarLauncher");
-    return entrypoint.build();
+  public ImmutableList<String> computeEntrypoint(List<String> jvmFlags) throws IOException {
+    try (JarFile jarFile = new JarFile(jarPath.toFile())) {
+      String mainClass =
+          jarFile.getManifest().getMainAttributes().getValue(Attributes.Name.MAIN_CLASS);
+      if (mainClass == null) {
+        throw new IllegalArgumentException(
+            "`Main-Class:` attribute for an application main class not defined in the input JAR's "
+                + "manifest (`META-INF/MANIFEST.MF` in the JAR).");
+      }
+      ImmutableList.Builder<String> entrypoint = ImmutableList.builder();
+      entrypoint.add("java");
+      entrypoint.addAll(jvmFlags);
+      entrypoint.add("-cp");
+      entrypoint.add(JarLayers.APP_ROOT.toString());
+      entrypoint.add(mainClass);
+      return entrypoint.build();
+    }
   }
 
   @Override

jib-cli/src/test/java/com/google/cloud/tools/jib/cli/jar/SpringBootExplodedProcessorTest.java

@@ -17,6 +17,7 @@
 package com.google.cloud.tools.jib.cli.jar;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
 
 import com.google.cloud.tools.jib.api.buildplan.AbsoluteUnixPath;
 import com.google.cloud.tools.jib.api.buildplan.FileEntriesLayer;
@@ -43,6 +44,8 @@ public class SpringBootExplodedProcessorTest {
   private static final String SPRING_BOOT_LAYERED_WITH_ALL_EMPTY_LAYERS_LISTED =
       "jar/spring-boot/springboot_layered_allEmptyLayers.jar";
   private static final String SPRING_BOOT_NOT_LAYERED = "jar/spring-boot/springboot_notLayered.jar";
+  private static final String SPRING_BOOT_NO_MANIFEST =
+      "jar/spring-boot/springboot_noMainClass_in_manifest.jar";
   private static final Integer JAR_JAVA_VERSION = 0; // any value
 
   @Rule public final TemporaryFolder temporaryFolder = new TemporaryFolder();
@@ -239,27 +242,53 @@ public void testCreateLayers_nonLayered() throws IOException, URISyntaxException
   }
 
   @Test
-  public void testComputeEntrypoint() {
+  public void testComputeEntrypoint() throws URISyntaxException, IOException {
+    Path sampleSpringBootJar = Paths.get(Resources.getResource(SPRING_BOOT_LAYERED).toURI());
     SpringBootExplodedProcessor bootProcessor =
         new SpringBootExplodedProcessor(
-            Paths.get("ignored"), Paths.get("ignored"), JAR_JAVA_VERSION);
+            sampleSpringBootJar, Paths.get("ignored"), JAR_JAVA_VERSION);
     ImmutableList<String> actualEntrypoint = bootProcessor.computeEntrypoint(new ArrayList<>());
     assertThat(actualEntrypoint)
         .isEqualTo(
-            ImmutableList.of("java", "-cp", "/app", "org.springframework.boot.loader.JarLauncher"));
+            ImmutableList.of(
+                "java", "-cp", "/app", "org.springframework.boot.loader.launch.JarLauncher"));
   }
 
   @Test
-  public void testComputeEntrypoint_withJvmFlags() {
+  public void testComputeEntrypoint_withJvmFlags() throws URISyntaxException, IOException {
+    Path sampleSpringBootJar = Paths.get(Resources.getResource(SPRING_BOOT_LAYERED).toURI());
     SpringBootExplodedProcessor bootProcessor =
         new SpringBootExplodedProcessor(
-            Paths.get("ignored"), Paths.get("ignored"), JAR_JAVA_VERSION);
+            sampleSpringBootJar, Paths.get("ignored"), JAR_JAVA_VERSION);
     ImmutableList<String> actualEntrypoint =
         bootProcessor.computeEntrypoint(ImmutableList.of("-jvm-flag"));
     assertThat(actualEntrypoint)
         .isEqualTo(
             ImmutableList.of(
-                "java", "-jvm-flag", "-cp", "/app", "org.springframework.boot.loader.JarLauncher"));
+                "java",
+                "-jvm-flag",
+                "-cp",
+                "/app",
+                "org.springframework.boot.loader.launch.JarLauncher"));
+  }
+
+  @Test
+  public void testComputeEntrypoint_noMainClass() throws URISyntaxException {
+    Path layeredSpringBootJar = Paths.get(Resources.getResource(SPRING_BOOT_NO_MANIFEST).toURI());
+    SpringBootExplodedProcessor bootProcessor =
+        new SpringBootExplodedProcessor(
+            layeredSpringBootJar, Paths.get("ignored"), JAR_JAVA_VERSION);
+
+    IllegalArgumentException exception =
+        assertThrows(
+            IllegalArgumentException.class,
+            () -> bootProcessor.computeEntrypoint(ImmutableList.of()));
+
+    assertThat(exception)
+        .hasMessageThat()
+        .isEqualTo(
+            "`Main-Class:` attribute for an application main class not defined in the input JAR's manifest "
+                + "(`META-INF/MANIFEST.MF` in the JAR).");
   }
 
   @Test