diff --git a/agent/src/main/java/com/alibaba/testable/constant/Const.java b/agent/src/main/java/com/alibaba/testable/constant/Const.java
index 9ab1b5c..f75f880 100644
--- a/agent/src/main/java/com/alibaba/testable/constant/Const.java
+++ b/agent/src/main/java/com/alibaba/testable/constant/Const.java
@@ -1,5 +1,8 @@
 package com.alibaba.testable.constant;
 
+import java.util.ArrayList;
+import java.util.List;
+
 /**
  * @author flin
  */
@@ -8,4 +11,9 @@ public class Const {
     public static final String DOT = ".";
     public static final String SLASH = "/";
 
+    public static final List<String> SYS_CLASSES = new ArrayList<String>();
+    static {
+        SYS_CLASSES.add("java/lang/StringBuilder");
+    }
+
 }
diff --git a/agent/src/main/java/com/alibaba/testable/model/TravelStatus.java b/agent/src/main/java/com/alibaba/testable/model/TravelStatus.java
new file mode 100644
index 0000000..2f55ddb
--- /dev/null
+++ b/agent/src/main/java/com/alibaba/testable/model/TravelStatus.java
@@ -0,0 +1,22 @@
+package com.alibaba.testable.model;
+
+/**
+ * @author flin
+ */
+
+public enum TravelStatus {
+
+    /**
+     * Initialized
+     */
+    INIT,
+    /**
+     * Processing member method replacement
+     */
+    MEM_REP,
+    /**
+     * Processing member operation replacement
+     */
+    NEW_REP
+
+}
diff --git a/agent/src/main/java/com/alibaba/testable/transformer/TestableClassTransformer.java b/agent/src/main/java/com/alibaba/testable/transformer/TestableClassTransformer.java
index 77acb45..f214eb5 100644
--- a/agent/src/main/java/com/alibaba/testable/transformer/TestableClassTransformer.java
+++ b/agent/src/main/java/com/alibaba/testable/transformer/TestableClassTransformer.java
@@ -1,11 +1,17 @@
 package com.alibaba.testable.transformer;
 
+import com.alibaba.testable.model.TravelStatus;
 import org.objectweb.asm.ClassReader;
 import org.objectweb.asm.ClassWriter;
 import org.objectweb.asm.Opcodes;
-import org.objectweb.asm.tree.ClassNode;
+import org.objectweb.asm.Type;
+import org.objectweb.asm.tree.*;
 
 import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+import static com.alibaba.testable.constant.Const.SYS_CLASSES;
 
 /**
  * @author flin
@@ -27,7 +33,63 @@ public class TestableClassTransformer implements Opcodes {
     }
 
     private void transform() {
+        List<String> methodNames = new ArrayList<String>();
+        for (MethodNode m : cn.methods) {
+            if (!"<init>".equals(m.name)) {
+                methodNames.add(m.name);
+            }
+        }
+        for (MethodNode m : cn.methods) {
+            transformMethod(m, methodNames);
+        }
+    }
 
+    private void transformMethod(MethodNode mn, List<String> methodNames) {
+        AbstractInsnNode[] instructions = mn.instructions.toArray();
+        TravelStatus status = TravelStatus.INIT;
+        String target = "";
+        int rangeStart = 0;
+        for (int i = 0; i < instructions.length; i++) {
+            if (instructions[i].getOpcode() == Opcodes.NEW) {
+                TypeInsnNode node = (TypeInsnNode)instructions[i];
+                if (!SYS_CLASSES.contains(node.desc) && node.desc.equals("com/alibaba/testable/BlackBox")) {
+                    target = node.desc;
+                    status = TravelStatus.NEW_REP;
+                    rangeStart = i;
+                }
+            }
+            if (instructions[i].getOpcode() == Opcodes.INVOKESPECIAL) {
+                MethodInsnNode node = (MethodInsnNode)instructions[i];
+                if (methodNames.contains(node.name) && cn.name.equals(node.owner)) {
+                    status = TravelStatus.MEM_REP;
+                }
+                if (TravelStatus.NEW_REP == status && "<init>".equals(node.name) && target.equals(node.owner)) {
+                    replaceNewOps(mn, instructions, rangeStart, i);
+                }
+            }
+        }
+    }
+
+    private void replaceNewOps(MethodNode mn, AbstractInsnNode[] instructions, int start, int end) {
+
+        InsnList il = new InsnList();
+        il.add(new LdcInsnNode(Type.getType("Lcom/alibaba/testable/BlackBox;")));
+        il.add(new InsnNode(ICONST_1));
+        il.add(new TypeInsnNode(ANEWARRAY, "java/lang/Object"));
+        il.add(new InsnNode(DUP));
+        il.add(new InsnNode(ICONST_0));
+        il.add(new LdcInsnNode("something"));
+        il.add(new InsnNode(AASTORE));
+        il.add(new MethodInsnNode(INVOKESTATIC, "testable_internal/n/e",
+            "w", "(Ljava/lang/Class;[Ljava/lang/Object;)Ljava/lang/Object;", false));
+        il.add(new TypeInsnNode(CHECKCAST, "com/alibaba/testable/BlackBox"));
+        mn.instructions.insert(instructions[end], il);
+
+        for (int z = start; z <= end; z++) {
+            mn.instructions.remove(instructions[z]);
+        }
+
+        mn.maxStack += 4;
     }
 
 }
diff --git a/agent/src/main/java/com/alibaba/testable/transformer/TestableFileTransformer.java b/agent/src/main/java/com/alibaba/testable/transformer/TestableFileTransformer.java
index 147a8da..47e8197 100644
--- a/agent/src/main/java/com/alibaba/testable/transformer/TestableFileTransformer.java
+++ b/agent/src/main/java/com/alibaba/testable/transformer/TestableFileTransformer.java
@@ -23,8 +23,8 @@ public class TestableFileTransformer implements ClassFileTransformer {
 
     public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
                             ProtectionDomain protectionDomain, byte[] classfileBuffer) {
-        if (isSystemClass(loader, className)) {
-            // Ignore system class
+        if (isSystemClass(loader, className) || loadedClassNames.contains(className)) {
+            // Ignore system class and duplicate class
             return null;
         }
 
@@ -36,6 +36,7 @@ public class TestableFileTransformer implements ClassFileTransformer {
         }
 
         try {
+            loadedClassNames.add(className);
             return new TestableClassTransformer(className).getBytes();
         } catch (IOException e) {
             return null;