My JavaScript code:

var foo = new Foo();

My Java code:

v8Runtime.registerJavaMethod(JavaCallback { _, _ -> Foo("Loan") }, "Foo")

Exception information:

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.loasan.testj2v8/com.loasan.testj2v8.MainActivity}: undefined:32: Unknown return type: class com.loasan.testj2v8.Engine$Foo
    var foo = new Foo()
              ^
    com.eclipsesource.v8.V8ScriptExecutionException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3725)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3893)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:140)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:100)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2342)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:269)