Fix WorkflowStub.fromTyped method (#500)

Author: meiliang86

src/main/java/com/uber/cadence/client/WorkflowStub.java

@@ -20,6 +20,8 @@
 import com.uber.cadence.QueryRejectCondition;
 import com.uber.cadence.WorkflowExecution;
 import com.uber.cadence.internal.common.QueryResponse;
+import java.lang.reflect.InvocationHandler;
+import java.lang.reflect.Proxy;
 import java.lang.reflect.Type;
 import java.util.Optional;
 import java.util.concurrent.CompletableFuture;
@@ -44,12 +46,20 @@ public interface WorkflowStub {
    * @return untyped workflow stub for the same workflow instance.
    */
   static <T> WorkflowStub fromTyped(T typed) {
-    if (!(typed instanceof Supplier)) {
+    if (!(typed instanceof Proxy)) {
       throw new IllegalArgumentException(
           "arguments must be created through WorkflowClient.newWorkflowStub");
     }
+
+    InvocationHandler handler = Proxy.getInvocationHandler(typed);
+
+    if (!(handler instanceof Supplier)) {
+      throw new IllegalArgumentException(
+          "arguments must be created through WorkflowClient.newWorkflowStub");
+    }
+
     @SuppressWarnings("unchecked")
-    Supplier<WorkflowStub> supplier = (Supplier<WorkflowStub>) typed;
+    Supplier<WorkflowStub> supplier = (Supplier<WorkflowStub>) handler;
     return supplier.get();
   }
 

src/main/java/com/uber/cadence/internal/sync/WorkflowInvocationHandler.java

@@ -38,12 +38,18 @@
 import java.lang.reflect.InvocationHandler;
 import java.lang.reflect.Method;
 import java.util.Optional;
+import java.util.function.Supplier;
 
 /**
  * Dynamic implementation of a strongly typed workflow interface that can be used to start, signal
  * and query workflows from external processes.
  */
-class WorkflowInvocationHandler implements InvocationHandler {
+class WorkflowInvocationHandler implements InvocationHandler, Supplier<WorkflowStub> {
+
+  @Override
+  public WorkflowStub get() {
+    return untyped;
+  }
 
   public enum InvocationType {
     SYNC,

src/test/java/com/uber/cadence/workflow/WorkflowTest.java

@@ -1395,6 +1395,16 @@ public void testStart() {
     assertEquals("1234", stubP4.query());
     assertEquals("12345", stubP5.query());
     assertEquals("123456", stubP6.query());
+
+    // Test execution from untyped stub.
+    workflowOptions =
+        newWorkflowOptionsBuilder(taskList).setWorkflowId(UUID.randomUUID().toString()).build();
+    TestMultiargsWorkflowsFunc stub2 =
+        workflowClient.newWorkflowStub(TestMultiargsWorkflowsFunc.class, workflowOptions);
+    WorkflowStub untypedStub = WorkflowStub.fromTyped(stub2);
+    untypedStub.start();
+    String result = untypedStub.getResult(String.class);
+    assertEquals("func", result);
   }
 
   @Test