So in Minecraft 1.7.10, the Minecraft session field is private and final. I can change it to public with an access transformer (which is easy) but I can't actually modify the session id because it's final. Is there a way to edit this or would I have to code a fully custom client?

You can set it to be accessible using reflection. This StackoOverflow post shows you how to do it:

http://stackoverflow.com/questions/3301635/change-private-static-final-field-using-java-reflection

 

But be aware that reflection uses quite a lot of cpu power...

So I used the same method as in the stackoverflow post but I get a field not found exception when I tell it to modify Minecraft.class.getField("session")

First of all, I suggest using either the ReflectionHelper or ObfuscationReflectionHelper classes - they make cleaner code, and change this:

 

try
{
    Field field = SomeClass.class.getDeclaredField("someFieldName");
    field.setAccessible(true);
    Object object = field.get(SOMECLASS_INSTANCE);
    //do stuff
}
catch (Exception e)
{
    try
    {
        Field field = SomeClass.class.getDeclaredField("field_SRG_NAME");
        field.setAccessible(true);
        Object object = field.get(SOMECLASS_INSTANCE);
        //do stuff
    }
    catch(Exception e2)
    {
        //Log exception
    }
}

 

into

 

Object object = ReflectionHelper.getPrivateValue(SomeClass.class, SOMECLASS_INSTANCE, "someFieldName", "field_SRG_NAME");

 

Much cleaner.

 

Example code

Field sessionField = ReflectionHelper.findField(Minecraft.class, "session", "field_71449_j");
ReflectionHelper.setPrivateValue(Field.class, sessionField, sessionField.getModifiers() & ~Modifier.FINAL, "modifiers");
ReflectionHelper.setPrivateValue(Minecraft.class, Minecraft.getMinecraft(), YOUR_SESSION_INSTANCE, "session", "field_71449_j");