RFR: 8228773: URLClassLoader constructors should include API note warning that the parent should not be null [v4]

[Alan Bateman]

On Fri, 30 May 2025 06:55:07 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

>> Can I please get a review of this doc-only change which proposes to add an `@apiNote` on the constructors of `URLClassLoader` and `SecureClassLoader` to explain the current implementation of these constructors? This addresses https://bugs.openjdk.org/browse/JDK-8228773?
>> 
>> As noted in that issue, this updated documentation is to help applications be aware that a `null` value which represents the bootstrap class loader when passed for `parent` class loader will mean that the constructed `URLClassLoader` may not be able to load all platform classes.
>> 
>> Specifically, only a specific set of modules are mapped (at JDK build time) to the bootstrap class loader. Some modules like `java.sql` are visible to the platform class loader but not to the bootstrap classloader. The distinction between these class loaders is explained in the API documentation of `ClassLoader` class https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/ClassLoader.html#builtinLoaders.
>> 
>> Using `null` (which represents a bootstrap class loader) for `parent` when constructing the `URLClassLoader` then means that the class loader would not be able to load some of these platform classes. For example, consider:
>> 
>> 
>> import java.net.*;
>> 
>> public class Test {
>>     public static void main(String[] args) throws Exception {
>>         System.out.println("testing against Java " + System.getProperty("java.version"));
>> 
>>         final ClassLoader cl = new URLClassLoader(new URL[0], null);
>>         final Class<?> klass = cl.loadClass("java.sql.ResultSet"); // load a platform class that belongs to the java.sql module
>>         System.out.println("loaded " + klass + " using classloader: " + klass.getClassLoader());
>> 
>>     }
>> }
>> 
>> which constructs the `URLClassLoader` with the bootstrap class loader as its parent and then attempts to load a platform class `java.sql.ResultSet`. Since this class' module is mapped to the platform class loader and not the bootstrap class loader, running this code against Java 9+ will result in a `ClassNotFoundException`:
>> 
>> 
>> testing against Java 24
>> Exception in thread "main" java.lang.ClassNotFoundException: java.sql.ResultSet
>>     at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:349)
>>     at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:557)
>>     at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:490)
>>     at Test.main(Test.java:8)
>> 
>> 
>> No new tests have been introduced and existing tes...
>
> Jaikiran Pai has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - replace system class loader reference with bootstrap class loader
>  - remove "the"
>  - {@code null} instead of null

Marked as reviewed by alanb (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/25517#pullrequestreview-2880392639