Use of -noverify when launching java apps

I have seen many apps that take instrument classes and take -javaagent as a param when loading also put a -noverify to the command line.

The Java doc says that -noverify turns off class verification.

However why would anyone want to turn off verification even if they are instrumenting classes?

Start-up time, I'd say. Verification that classes are correct takes some time when the class is loaded. Since classes might be loaded in a lazy fashion (not on app start, but when being used for the first time), this might cause unexpected and undesired runtime delays.

Actually the class does not need to be checked in general. The compiler will not emit any invalid bytecode or class construct. The reason for verification is that the class may be build on one system, get hosted online and is transmitted to you through the unprotected internet. On this path, a malicious attacker might modify the bytecode and create something the compiler might never create; something that can crash the JVM or possibly circumvents security restrictions. Thus the class is verified before it is used. If this is a local application, there is usually no need to check the bytecode again.

When it is used in conjunction with -javaagent, it is most likely not for performance reasons, but because the agent intentionally creates "invalid" bytecode.

It should be noted that invalid bytecode might still execute fine, because some of the verification rules are quite strict. For instance, this must not be accessed in a constructor before the super-constructor was called, because the variables are not initialized at this point. But there still might be other things you want to do (see the JRebel example). Then, you use -noverify to circumvent that rule.

Debugging! In fact that's what I'm doing now, and how I stumbled across this question. At Terracotta we do a lot of bytecode instrumentation, and sometimes it helps to turn off the verifier as we debug our class adapters, so we can see where exactly they fail at runtime.

You're right, we want the verifier to remain on in production.

Using JRebel without -noverify will give this warning on startup:

JRebel: '-noverify' missing, changing/adding/removing constructors will not be enabled!

So appears that -noverify allows bytecode re-instrumentation to do some things which would otherwise not be possible.