Add a test for AgentMain which verifies that all classes from the spec are actually instrumented

pom.xml

@@ -46,6 +46,7 @@
             </includes>
             <excludes>
               <exclude>**/TransformerTest.java</exclude>
+              <exclude>**/AgentMainTest.java</exclude>
             </excludes>
             <argLine>
               -javaagent:"${project.build.directory}/file-leak-detector-${project.version}-jar-with-dependencies.jar"
@@ -60,9 +61,8 @@
               </goals>
               <configuration>
                 <includes>
-                  <include>
-                    **/TransformerTest.java
-                  </include>
+                  <include>**/TransformerTest.java</include>
+                  <include>**/AgentMainTest.java</include>
                 </includes>
                 <excludes>
                   <exclude>**/instrumented/*.java</exclude>
@@ -142,6 +142,12 @@
       <version>4.13.1</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <version>3.2.4</version>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <profiles>

src/test/java/org/kohsuke/file_leak_detector/AgentMainTest.java

@@ -0,0 +1,74 @@
+package org.kohsuke.file_leak_detector;
+
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyBoolean;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import java.lang.instrument.ClassFileTransformer;
+import java.lang.instrument.Instrumentation;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import org.junit.Test;
+import org.kohsuke.file_leak_detector.transform.ClassTransformSpec;
+import org.mockito.Mockito;
+import org.mockito.invocation.InvocationOnMock;
+import org.mockito.stubbing.Answer;
+
+public class AgentMainTest {
+    @Test
+    public void noDuplicateSpecs() {
+        final List<ClassTransformSpec> specs = AgentMain.createSpec();
+
+        Set<String> seenClasses = new HashSet<>();
+        for (ClassTransformSpec spec : specs) {
+            assertTrue("Did have duplicate spec for class " + spec.name, seenClasses.add(spec.name));
+        }
+    }
+
+    @Test
+    public void testPreMain() throws Exception {
+        final List<ClassTransformSpec> specs = AgentMain.createSpec();
+
+        final Set<String> seenClasses = new HashSet<>();
+        for (ClassTransformSpec spec : specs) {
+            seenClasses.add(spec.name);
+        }
+
+        Instrumentation instrumentation = mock(Instrumentation.class);
+        Mockito.doAnswer(new Answer<Object>() {
+            @Override
+            public Object answer(InvocationOnMock invocationOnMock) throws Throwable {
+                for (Object obj : invocationOnMock.getArguments()) {
+                    Class<?> clazz = (Class<?>) obj;
+                    String name = clazz.getName().replace(".", "/");
+                    assertTrue(
+                            "Tried to transform a class wihch is not contained in the specs: "
+                                    + name
+                                    + " ("
+                                    + clazz
+                                    + "), having remaining classes: "
+                                    + seenClasses,
+                            seenClasses.remove(name));
+                }
+                return null;
+            }
+        }).when(instrumentation).retransformClasses((Class<?>) any());
+
+        AgentMain.premain(null, instrumentation);
+
+        verify(instrumentation, times(1)).addTransformer((ClassFileTransformer) any(), anyBoolean());
+        verify(instrumentation, times(1)).retransformClasses((Class<?>) any());
+
+        // the following are not available in all JVMs
+        seenClasses.remove("sun/nio/ch/SocketChannelImpl");
+        seenClasses.remove("java/net/AbstractPlainSocketImpl");
+
+        assertTrue(
+                "Had classes in the spec which were not instrumented: " + seenClasses,
+                seenClasses.isEmpty());
+    }
+}