Enable instrumenting old java bytecode

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/DDTransformers.java

@@ -0,0 +1,26 @@
+package datadog.trace.agent.tooling;
+
+import net.bytebuddy.agent.builder.AgentBuilder;
+import net.bytebuddy.asm.TypeConstantAdjustment;
+import net.bytebuddy.description.type.TypeDescription;
+import net.bytebuddy.dynamic.DynamicType;
+import net.bytebuddy.utility.JavaModule;
+
+public class DDTransformers {
+
+  private static final AgentBuilder.Transformer CONSTANT_ADJUSTER =
+      new AgentBuilder.Transformer() {
+        @Override
+        public DynamicType.Builder<?> transform(
+            DynamicType.Builder<?> builder,
+            TypeDescription typeDescription,
+            ClassLoader classLoader,
+            JavaModule javaModule) {
+          return builder.visit(TypeConstantAdjustment.INSTANCE);
+        }
+      };
+
+  public static AgentBuilder.Transformer defaultTransformers() {
+    return CONSTANT_ADJUSTER;
+  }
+}

dd-java-agent/testing/src/test/groovy/InstrumentOldBytecode.groovy

@@ -0,0 +1,9 @@
+import com.ibm.as400.resource.ResourceLevel
+import datadog.trace.agent.test.AgentTestRunner
+
+class InstrumentOldBytecode extends AgentTestRunner {
+  def "can instrument old bytecode"() {
+    expect:
+    new ResourceLevel().toString() == "instrumented"
+  }
+}

dd-java-agent/testing/src/test/java/IBMResourceLevelInstrumentation.java

@@ -0,0 +1,31 @@
+import static net.bytebuddy.matcher.ElementMatchers.named;
+
+import com.google.auto.service.AutoService;
+import datadog.trace.agent.tooling.DDAdvice;
+import datadog.trace.agent.tooling.DDTransformers;
+import datadog.trace.agent.tooling.Instrumenter;
+import net.bytebuddy.agent.builder.AgentBuilder;
+import net.bytebuddy.asm.Advice;
+
+@AutoService(Instrumenter.class)
+public class IBMResourceLevelInstrumentation extends Instrumenter.Configurable {
+  public IBMResourceLevelInstrumentation() {
+    super(IBMResourceLevelInstrumentation.class.getName());
+  }
+
+  @Override
+  protected AgentBuilder apply(AgentBuilder agentBuilder) {
+    return agentBuilder
+        .type(named("com.ibm.as400.resource.ResourceLevel"))
+        .transform(DDTransformers.defaultTransformers())
+        .transform(DDAdvice.create().advice(named("toString"), ToStringAdvice.class.getName()))
+        .asDecorator();
+  }
+
+  public static class ToStringAdvice {
+    @Advice.OnMethodExit(suppress = Throwable.class)
+    static void toStringReplace(@Advice.Return(readOnly = false) String ret) {
+      ret = "instrumented";
+    }
+  }
+}

dd-java-agent/testing/testing.gradle

@@ -10,4 +10,7 @@ dependencies {
 
   compile project(':dd-trace-ot')
   compile project(':dd-java-agent:agent-tooling')
+
+  // test instrumenting java 1.1 bytecode
+  testCompile group: 'net.sf.jt400', name: 'jt400', version: '6.1'
 }