Changed block cache to soft references (should help a lot with memory usage)

By: Dinnerbone <dinnerbone@dinnerbone.com>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftChunk.java

@@ -10,10 +10,11 @@ import org.bukkit.Chunk;
 import org.bukkit.World;
 import org.bukkit.block.Block;
 import org.bukkit.craftbukkit.block.CraftBlock;
+import org.bukkit.craftbukkit.util.SoftMap;
 
 public class CraftChunk implements Chunk {
     private WeakReference<net.minecraft.server.Chunk> weakChunk;
-    private final HashMap<Integer, Block> cache = new HashMap<Integer, Block>();
+    private final SoftMap<Integer, Block> cache = new SoftMap<Integer, Block>();
     private WorldServer worldServer;
     private int x;
     private int z;

paper-server/src/main/java/org/bukkit/craftbukkit/util/SoftMap.java

@@ -0,0 +1,200 @@
+package org.bukkit.craftbukkit.util;
+
+import java.util.Map;
+import java.util.AbstractMap;
+import java.util.LinkedList;
+import java.util.HashMap;
+import java.util.Set;
+import java.util.Collection;
+import java.util.Iterator;
+
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.SoftReference;
+
+/**
+ * Creates a map that uses soft reference.  This indicates to the garbage collector
+ * that they can be removed if necessary 
+ *
+ * A minimum number of strong references can be set.  These most recent N objects added 
+ * to the map will not be removed by the garbage collector.
+ *
+ * Objects will never be removed if they are referenced strongly from somewhere else
+
+ * Note: While data corruption won't happen, the garbage collector is potentially async
+ *       This could lead to the return values from containsKey() and similar methods being 
+ *       out of date by the time they are used.  The class could return null when the object
+ *       is retrieved by a .get() call directly after a .containsKey() call returned true
+ *
+ * @author raphfrk
+ */
+
+public class SoftMap<K,V> {
+
+    private final HashMap<K,SoftMapReference<K,V>> map = new HashMap<K,SoftMapReference<K,V>>();
+    private final ReferenceQueue<SoftMapReference> queue = new ReferenceQueue<SoftMapReference>();
+    private final LinkedList<V> strongReferenceQueue = new LinkedList<V>();
+    private final int strongReferenceSize;
+
+    public SoftMap() {
+        this(20);
+    }
+
+    public SoftMap(int size) {
+        strongReferenceSize = size;
+    }
+
+    // When a soft reference is deleted by the garbage collector, it is set to reference null
+    // and added to the queue
+    //
+    // However, these null references still exist in the HashMap as keys.  This method removes these keys.
+    // 
+    // It is called whenever there is a method call of the map.
+
+    private void emptyQueue() {
+        SoftMapReference ref;
+        while((ref=(SoftMapReference)queue.poll()) != null) {
+            map.remove(ref.key);
+        }
+    }
+
+    public void clear() {
+         strongReferenceQueue.clear();
+         map.clear();
+         emptyQueue();
+    }
+
+    // Shouldn't support this, since the garbage collection is async
+
+    public boolean containsKey(K key) {
+        emptyQueue();
+        return map.containsKey(key);
+    }
+
+    // Shouldn't support this, since the garbage collection is async
+
+    public boolean containsValue(V value) {
+        emptyQueue();
+        return map.containsValue(value);
+    }
+
+    // Shouldn't support this since it would create strong references to all the entries
+
+    public Set entrySet() {
+        emptyQueue();
+        throw new UnsupportedOperationException("SoftMap does not support this operation, since it creates potentially stong references");
+    }
+
+    // Doesn't support these either
+
+    public boolean equals(Object o) {
+         emptyQueue();
+         throw new UnsupportedOperationException("SoftMap doesn't support equals checks");
+    }
+
+    // This operation returns null if the entry is not in the map
+
+    public V get(K key) {
+        emptyQueue();
+        return fastGet(key);
+    }
+
+    private V fastGet(K key) {
+        SoftMapReference<K,V> ref = map.get(key);
+        if(ref==null) {
+            return null;
+        }
+        V value = ref.get();
+        if(value!=null) {
+            strongReferenceQueue.addFirst(value);
+            if(strongReferenceQueue.size() > strongReferenceSize) {
+                strongReferenceQueue.removeLast();
+            }
+        }
+        return value;
+    }
+
+    // Doesn't support this either
+
+    public int hashCode() {
+         emptyQueue();
+         throw new UnsupportedOperationException("SoftMap doesn't support hashCode");
+    }
+
+    // This is another risky method, since again, garbage collection is async
+
+    public boolean isEmpty() {
+        emptyQueue();
+        return map.isEmpty();
+    } 
+
+    // Return all the keys, again could go out of date
+
+    public Set keySet() {
+        emptyQueue();
+        return map.keySet();
+    }
+
+    // Adds the mapping to the map
+
+    public V put(K key, V value) {
+        emptyQueue();
+        V old = fastGet(key); 
+        fastPut(key, value);
+        return old;
+    }
+
+    private void fastPut(K key, V value) {
+        map.put(key, new SoftMapReference<K,V>(key, value, queue));
+        strongReferenceQueue.addFirst(value);
+        if(strongReferenceQueue.size() > strongReferenceSize) {
+            strongReferenceQueue.removeLast();
+        }
+    }
+
+    // Adds the mappings to the map
+
+    public void putAll(Map other) {
+        emptyQueue();
+        Iterator<K> itr = other.keySet().iterator();
+        while(itr.hasNext()) {
+            K key = itr.next();
+            fastPut(key, (V)other.get(key));
+        }
+    }
+
+    // Remove object
+
+    public V remove(K key) {
+        emptyQueue();
+        SoftMapReference<K,V> ref = map.remove(key);
+        if(ref != null) {
+            return ref.get();
+        }
+        return null;
+    }
+
+    // Returns size, could go out of date
+
+    public int size() {
+        emptyQueue();
+        return map.size();
+    } 
+
+    // Shouldn't support this since it would create strong references to all the entries
+
+    public Collection values() {
+        emptyQueue();
+        throw new UnsupportedOperationException("SoftMap does not support this operation, since it creates potentially stong references");
+    }
+
+
+    private static class SoftMapReference<K,V> extends SoftReference<V> {
+        K key;
+
+        SoftMapReference(K key, V value, ReferenceQueue queue) {
+            super(value, queue);
+            this.key = key;
+        }
+    }
+
+}