Is there restriction about Java package name length?

Question

Is there a restriction for the name length of a Java package name?

Like package com.abc.xxx.xxxx.xxxx

Answer 1

In class files there is a practical limit imposed by the fact that the full qualified class name is stored as a (modified) UFT-8 sequence which can have a maximum length of 65535 bytes as the length is store as an unsigned short.

Storing the signature of a Generic class might require an even shorter package name. And it will be even more limited if you try to create a method taking the type as a parameter multiple times as the signature will contain the qualified type name for every parameter and the return type in a single sequence, but it is still limited to 65535 UTF-8 bytes in total.

Answer 2

According to the Java language specification (3.8):

"An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter."

However, as @Holger points out, the JVM specification places an upper limit of 2^16 - 1 on string values that can be represented in a classfile constant pool. (See JVM spec 4.4.7)

Package names are part of fully qualified class names, and the latter are represented as classfile constant pool entries. So, the practical upper limit is on the length of the fully-qualified classname, not just the package name.

Hypothetically, if someone were to create a Java implementation that didn't use the standard classfile format, there is another limit ... the length of a Java String, which is 2^31 - 1 characters. (Note that the maximum length of a String is not specified in the JLS. It is the Java class libraries and the javadocs that specify this.)

However, the reality is that these implementation limits are so large that they should not present practical concerns.