Fixes #1262: Add diagnostic method pooledCount() in RecyclerPool

src/main/java/com/fasterxml/jackson/core/util/RecyclerPool.java

@@ -111,6 +111,25 @@ default boolean clear() {
         return false;
     }
 
+    /**
+     * Diagnostic method for obtaining an estimate of number of pooled items
+     * this pool contains, available for recycling.
+     * Note that in addition to this information possibly not being available
+     * (denoted by return value of {@code -1}) even when available this may be
+     * just an approximation.
+     *<p>
+     * Default method implementation simply returns {@code -1} and is meant to be
+     * overridden by concrete sub-classes.
+     *
+     * @return Number of pooled entries available from this pool, if available;
+     *    {@code -1} if not.
+     *
+     * @since 2.18
+     */
+    default int pooledCount() {
+        return -1;
+    }
+
     /*
     /**********************************************************************
     /* Partial/base RecyclerPool implementations
@@ -150,6 +169,12 @@ public void releasePooled(P pooled) {
              // nothing to do, relies on ThreadLocal
         }
 
+        // No way to actually even estimate...
+        @Override
+        public int pooledCount() {
+            return -1;
+        }
+
         // Due to use of ThreadLocal no tracking available; cannot clear
         @Override
         public boolean clear() {
@@ -181,6 +206,11 @@ public void releasePooled(P pooled) {
              // nothing to do, there is no underlying pool
         }
 
+        @Override
+        public int pooledCount() {
+            return 0;
+        }
+
         /**
          * Although no pooling occurs, we consider clearing to succeed,
          * so returns always {@code true}.
@@ -262,6 +292,11 @@ public void releasePooled(P pooled) {
             pool.offerLast(pooled);
         }
 
+        @Override
+        public int pooledCount() {
+            return pool.size();
+        }
+
         @Override
         public boolean clear() {
             pool.clear();
@@ -322,21 +357,30 @@ public void releasePooled(P pooled) {
             }
         }
 
-        protected static class Node<P> {
-            final P value;
-            Node<P> next;
-
-            Node(P value) {
-                this.value = value;
+        @Override
+        public int pooledCount() {
+            int count = 0;
+            for (Node<P> curr = head.get(); curr != null; curr = curr.next) {
+                ++count;
             }
+            return count;
         }
-
+        
         // Yes, we can clear it
         @Override
         public boolean clear() {
             head.set(null);
             return true;
         }
+
+        protected static class Node<P> {
+            final P value;
+            Node<P> next;
+
+            Node(P value) {
+                this.value = value;
+            }
+        }
     }
 
     /**
@@ -385,6 +429,11 @@ public void releasePooled(P pooled) {
             pool.offer(pooled);
         }
 
+        @Override
+        public int pooledCount() {
+            return pool.size();
+        }
+
         @Override
         public boolean clear() {
             pool.clear();

src/test/java/com/fasterxml/jackson/core/io/BufferRecyclerPoolTest.java

@@ -130,6 +130,11 @@ public void releasePooled(BufferRecycler r) {
             bufferRecycler = r;
         }
 
+        @Override
+        public int pooledCount() {
+            return (bufferRecycler == null) ? 0 : 1;
+        }
+
         @Override
         public boolean clear() {
             bufferRecycler = null;

src/test/java/com/fasterxml/jackson/core/util/JsonBufferRecyclersTest.java

@@ -16,37 +16,41 @@ class JsonBufferRecyclersTest extends JUnit5TestBase
 
     @Test
     void parserWithThreadLocalPool() throws Exception {
-        _testParser(JsonRecyclerPools.threadLocalPool());
+        _testParser(JsonRecyclerPools.threadLocalPool(), -1, -1);
     }
 
     @Test
     void parserWithNopLocalPool() throws Exception {
-        _testParser(JsonRecyclerPools.nonRecyclingPool());
+        _testParser(JsonRecyclerPools.nonRecyclingPool(), 0, 0);
     }
 
     @Test
     void parserWithDequeuPool() throws Exception {
-        _testParser(JsonRecyclerPools.newConcurrentDequePool());
-        _testParser(JsonRecyclerPools.sharedConcurrentDequePool());
+        _testParser(JsonRecyclerPools.newConcurrentDequePool(), 0, 1);
+        _testParser(JsonRecyclerPools.sharedConcurrentDequePool(), null, null);
     }
 
     @Test
     void parserWithLockFreePool() throws Exception {
-        _testParser(JsonRecyclerPools.newLockFreePool());
-        _testParser(JsonRecyclerPools.sharedLockFreePool());
+        _testParser(JsonRecyclerPools.newLockFreePool(), 0, 1);
+        _testParser(JsonRecyclerPools.sharedLockFreePool(), null, null);
     }
 
     @Test
     void parserWithBoundedPool() throws Exception {
-        _testParser(JsonRecyclerPools.newBoundedPool(5));
-        _testParser(JsonRecyclerPools.sharedBoundedPool());
+        _testParser(JsonRecyclerPools.newBoundedPool(5), 0, 1);
+        _testParser(JsonRecyclerPools.sharedBoundedPool(), null, null);
     }
 
-    private void _testParser(RecyclerPool<BufferRecycler> pool) throws Exception
+    private void _testParser(RecyclerPool<BufferRecycler> pool,
+            Integer expSizeBefore, Integer expSizeAfter) throws Exception
     {
         JsonFactory jsonF = JsonFactory.builder()
                 .recyclerPool(pool)
                 .build();
+        if (expSizeBefore != null) {
+            assertEquals(expSizeBefore, pool.pooledCount());
+        }
 
         JsonParser p = jsonF.createParser(a2q("{'a':123,'b':'foobar'}"));
 
@@ -62,44 +66,53 @@ private void _testParser(RecyclerPool<BufferRecycler> pool) throws Exception
         assertToken(JsonToken.END_OBJECT, p.nextToken());
 
         p.close();
+
+        if (expSizeAfter != null) {
+            assertEquals(expSizeAfter, pool.pooledCount());
+        }
     }
 
     // // Generators with RecyclerPools:
 
     @Test
     void generatorWithThreadLocalPool() throws Exception {
-        _testGenerator(JsonRecyclerPools.threadLocalPool());
+        _testGenerator(JsonRecyclerPools.threadLocalPool(), -1, -1);
     }
 
     @Test
     void generatorWithNopLocalPool() throws Exception {
-        _testGenerator(JsonRecyclerPools.nonRecyclingPool());
+        _testGenerator(JsonRecyclerPools.nonRecyclingPool(), 0, 0);
     }
 
     @Test
     void generatorWithDequeuPool() throws Exception {
-        _testGenerator(JsonRecyclerPools.newConcurrentDequePool());
-        _testGenerator(JsonRecyclerPools.sharedConcurrentDequePool());
+        _testGenerator(JsonRecyclerPools.newConcurrentDequePool(), 0, 1);
+        _testGenerator(JsonRecyclerPools.sharedConcurrentDequePool(), null, null);
     }
 
     @Test
     void generatorWithLockFreePool() throws Exception {
-        _testGenerator(JsonRecyclerPools.newLockFreePool());
-        _testGenerator(JsonRecyclerPools.sharedLockFreePool());
+        _testGenerator(JsonRecyclerPools.newLockFreePool(), 0, 1);
+        _testGenerator(JsonRecyclerPools.sharedLockFreePool(), null, null);
     }
 
     @Test
     void generatorWithBoundedPool() throws Exception {
-        _testGenerator(JsonRecyclerPools.newBoundedPool(5));
-        _testGenerator(JsonRecyclerPools.sharedBoundedPool());
+        _testGenerator(JsonRecyclerPools.newBoundedPool(5), 0, 1);
+        _testGenerator(JsonRecyclerPools.sharedBoundedPool(), null, null);
     }
-
-    private void _testGenerator(RecyclerPool<BufferRecycler> pool) throws Exception
+
+    private void _testGenerator(RecyclerPool<BufferRecycler> pool,
+            Integer expSizeBefore, Integer expSizeAfter) throws Exception
     {
         JsonFactory jsonF = JsonFactory.builder()
                 .recyclerPool(pool)
                 .build();
 
+        if (expSizeBefore != null) {
+            assertEquals(expSizeBefore, pool.pooledCount());
+        }
+
         StringWriter w = new StringWriter();
         JsonGenerator g = jsonF.createGenerator(w);
 
@@ -110,6 +123,10 @@ private void _testGenerator(RecyclerPool<BufferRecycler> pool) throws Exception
 
         g.close();
 
+        if (expSizeAfter != null) {
+            assertEquals(expSizeAfter, pool.pooledCount());
+        }
+
         assertEquals(a2q("{'a':-42,'b':'barfoo'}"), w.toString());
     }