A compromise approach on circularity checking

[David M. Lloyd]

I'd like to propose a patch [1] which allows run-time circularity 
checking to be disabled on an opt-out basis.  This allows an approach 
wherein data can be collected as to the usefulness of circularity 
checking, without locking users into it (at least in the near term).

Justification:

• In many environments, including large enterprises, or large open- and 
closed-source projects which may consume or assemble many dependencies, 
it is possible that a module which builds without cycles may fail due to 
a cycle at assembly time.
• By making this change opt-out, most users need not worry about it; if 
assembly problems occur at the end of the development or test process, 
however, users have an "escape hatch" at their disposal.
• If it becomes clear that allowing cycles at run time is indeed a valid 
requirement, then cycle checking at run time can be disabled permanently 
in Java 10 or a later release.

Drawbacks:

None that I am aware of.

Patch information:

The approach of this patch simply looks for a system property and, 
depending on the value of this property, enables or disables cycle 
checking.  I relinquish any copyright claim to the patch in this mail. 
Please do not get bogged down by any formatting problems introduced by 
the mailing list; the purpose of directly including the patch is to give 
a clear, unambiguous subject for discussion.  I can provide a proper 
webrev (or whatever other form is requested) if needed.

Other than the system property (which is just one possible approach), no 
API or specification changes should be needed.  This version uses the 
property "jdk.module.detect-cycles" defaulting to "true".

[1]:

diff --git 
a/jdk/src/java.base/share/classes/java/lang/module/Resolver.java 
b/jdk/src/java.base/share/classes/java/lang/module/Resolver.java
index a723e63..2451358 100644
--- a/jdk/src/java.base/share/classes/java/lang/module/Resolver.java
+++ b/jdk/src/java.base/share/classes/java/lang/module/Resolver.java
@@ -351,6 +351,8 @@ final class Resolver {
          }
      }

+    private static final boolean DETECT_CYCLES = 
Boolean.parseBoolean(System.getProperty("jdk.module.detect-cycles", 
"true"));
+
      /**
       * Execute post-resolution checks and returns the module graph of 
resolved
       * modules as {@code Map}. The resolved modules will be in the given
@@ -362,7 +364,7 @@ final class Resolver {
                                                      boolean check)
      {
          if (check) {
-            detectCycles();
+            if (DETECT_CYCLES) detectCycles();
              checkHashes();
          }



-- 
- DML