[GitHub] [lucene-solr] uschindler commented on pull request #1779: LUCENE-9478: Prevent DWPTDeleteQueue from referencing itself and leaking memory

[GitBox <gi...@apache.org>]

uschindler commented on pull request #1779:
URL: https://github.com/apache/lucene-solr/pull/1779#issuecomment-679311573


   This is a limitation of the Java Compiler javac. Whenever you access an instance field from a class (final or not doe snot matter), it created a lamda bount to this. The lambda method then does a getfield and invokes method. Here is the Bytecode of the lambda before this change:
   
   ```
     private long lambda$advanceQueue$1();
       Code:
          0: aload_0
          1: getfield      #20                 // Field nextSeqNo:Ljava/util/concurrent/atomic/AtomicLong;
          4: invokevirtual #75                 // Method java/util/concurrent/atomic/AtomicLong.get:()J
          7: lconst_1
          8: lsub
          9: lreturn
   ```
   
   As you see it's a non-static method. The generated lambda therefor has a reference to this (methodhandle bound to `this`).
   
   After your patch, the lambda is static:
   
   ```
     private static long lambda$getPrevMaxSeqIdSupplier$1(java.util.concurrent.atomic.AtomicLong);
       Code:
          0: aload_0
          1: invokevirtual #75                 // Method java/util/concurrent/atomic/AtomicLong.get:()J
          4: lconst_1
          5: lsub
          6: lreturn
   ```
   
   The generated anonymous class will have just bw bound tothe AtomicLong and not `this`.
   
   By analyzing the code we see that issue is fixed, test is not necessarily needed, but it's hard to check the bytecode :-)
   
   +1 for the fix!


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