Replace reflection with test accessor method

Author: JonasKunz

instrumentation/internal/internal-class-loader/javaagent/src/test/java/io/opentelemetry/javaagent/instrumentation/internal/classloader/DefineClassInstrumentationTest.java

@@ -13,7 +13,6 @@
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
-import java.lang.reflect.Field;
 import java.security.ProtectionDomain;
 import org.apache.commons.compress.utils.IOUtils;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -63,7 +62,7 @@ void ensureDefineClassInstrumented(int argCount) throws IOException {
     mockHandler.beforeDefineClass(cl, className, bytecode, 0, bytecode.length);
     Mockito.reset(mockHandler);
 
-    DefineClassHelper.Handler originalHandler = forceSetDefineClassHelper(mockHandler);
+    DefineClassHelper.Handler originalHandler = DefineClassHelper.internalSetHandlerForTests(mockHandler);
     String expectedClassName;
     try {
       switch (argCount) {
@@ -83,25 +82,12 @@ void ensureDefineClassInstrumented(int argCount) throws IOException {
           throw new IllegalStateException();
       }
     } finally {
-      forceSetDefineClassHelper(originalHandler);
+      DefineClassHelper.internalSetHandlerForTests(originalHandler);
     }
 
     verify(mockHandler)
         .beforeDefineClass(
             same(cl), eq(expectedClassName), eq(bytecode), eq(0), eq(bytecode.length));
     verify(mockHandler).afterDefineClass(eq(null));
   }
-
-  private static DefineClassHelper.Handler forceSetDefineClassHelper(
-      DefineClassHelper.Handler newHandler) {
-    try {
-      Field handler = DefineClassHelper.class.getDeclaredField("handler");
-      handler.setAccessible(true);
-      DefineClassHelper.Handler previous = (DefineClassHelper.Handler) handler.get(null);
-      handler.set(null, newHandler);
-      return previous;
-    } catch (Exception e) {
-      throw new IllegalStateException(e);
-    }
-  }
 }

javaagent-bootstrap/src/main/java/io/opentelemetry/javaagent/bootstrap/DefineClassHelper.java

@@ -70,5 +70,17 @@ public static void internalSetHandler(Handler handler) {
     DefineClassHelper.handler = handler;
   }
 
+  /**
+   * Only for testing. In contrast to {@link #internalSetHandler(Handler)} allows replacing the handler
+   * if it already has been set.
+   * @param handler the handler to set
+   * @return the previously active handler
+   */
+  public static Handler internalSetHandlerForTests(Handler handler) {
+    Handler oldHandler = DefineClassHelper.handler;
+    DefineClassHelper.handler = handler;
+    return oldHandler;
+  }
+
   private DefineClassHelper() {}
 }