Less heap memory is needed when multiple databases are open at the same time.

h2/src/main/org/h2/command/Command.java

@@ -17,6 +17,7 @@ import org.h2.message.Trace;
 import org.h2.message.TraceObject;
 import org.h2.result.LocalResult;
 import org.h2.result.ResultInterface;
+import org.h2.util.MemoryUtils;
 import org.h2.util.ObjectArray;
 
 /**
@@ -193,7 +194,7 @@ public abstract class Command implements CommandInterface {
     public int executeUpdate() throws SQLException {
         long start = startTime = System.currentTimeMillis();
         Database database = session.getDatabase();
-        database.allocateReserveMemory();
+        MemoryUtils.allocateReserveMemory();
         Object sync = database.isMultiThreaded() ? (Object) session : (Object) database;
         session.waitIfExclusiveModeEnabled();
         synchronized (sync) {
@@ -205,7 +206,7 @@ public abstract class Command implements CommandInterface {
                     try {
                         return update();
                     } catch (OutOfMemoryError e) {
-                        database.freeReserveMemory();
+                        MemoryUtils.freeReserveMemory();
                         throw Message.convert(e);
                     } catch (SQLException e) {
                         if (e.getErrorCode() == ErrorCode.CONCURRENT_UPDATE_1) {

h2/src/main/org/h2/engine/Database.java

@@ -154,13 +154,12 @@ public class Database implements DataHandler {
     private boolean multiVersion;
     private DatabaseCloser closeOnExit;
     private Mode mode = Mode.getInstance(Mode.REGULAR);
-    // TODO change in version 1.1
+    // TODO change in version 1.2
     private boolean multiThreaded;
     private int maxOperationMemory = SysProperties.DEFAULT_MAX_OPERATION_MEMORY;
     private boolean lobFilesInDirectories = SysProperties.LOB_FILES_IN_DIRECTORIES;
     private SmallLRUCache lobFileListCache = new SmallLRUCache(128);
     private boolean autoServerMode;
-    private Object reserveMemory;
     private Server server;
     private HashMap linkConnections;
     private TempFileDeleter tempFileDeleter = TempFileDeleter.getInstance();
@@ -2022,7 +2021,8 @@ public class Database implements DataHandler {
     }
 
     public void setMultiThreaded(boolean multiThreaded) throws SQLException {
-        if (multiThreaded && multiVersion) {
+        if (multiThreaded && multiVersion && this.multiThreaded != multiThreaded) {
+            // currently the combination of MVCC and MULTI_THREADED is not supported
             throw Message.getSQLException(ErrorCode.CANNOT_CHANGE_SETTING_WHEN_OPEN_1, "MVCC & MULTI_THREADED");
         }
         this.multiThreaded = multiThreaded;
@@ -2060,23 +2060,6 @@ public class Database implements DataHandler {
     public boolean isSysTableLocked() {
         return meta.isLockedExclusively();
     }
-
-    /**
-     * Allocate a little main memory that is freed up when if no memory is
-     * available, so that rolling back a large transaction is easier.
-     */
-    public void allocateReserveMemory() {
-        if (reserveMemory == null) {
-            reserveMemory = new byte[SysProperties.RESERVE_MEMORY];
-        }
-    }
-    
-    /**
-     * Free up the reserve memory.
-     */
-    public void freeReserveMemory() {
-        reserveMemory = null;
-    }
     
     /**
      * Open a new connection or get an existing connection to another database.

h2/src/main/org/h2/util/MemoryUtils.java

@@ -6,6 +6,8 @@
  */
 package org.h2.util;
 
+import org.h2.constant.SysProperties;
+
 /**
  * This is a utility class with functions to measure the free and used memory.
  */
@@ -14,6 +16,7 @@ public class MemoryUtils {
     private static long lastGC;
     private static final int GC_DELAY = 50;
     private static final int MAX_GC = 8;
+    private static volatile byte[] reserveMemory;
     
     private MemoryUtils() {
         // utility class
@@ -59,5 +62,22 @@ public class MemoryUtils {
             }
         }
     }
+    
+    /**
+     * Allocate a little main memory that is freed up when if no memory is
+     * available, so that rolling back a large transaction is easier.
+     */
+    public static void allocateReserveMemory() {
+        if (reserveMemory == null) {
+            reserveMemory = new byte[SysProperties.RESERVE_MEMORY];
+        }
+    }
+    
+    /**
+     * Free up the reserve memory.
+     */
+    public static void freeReserveMemory() {
+        reserveMemory = null;
+    }    
 
 }

h2/src/test/org/h2/test/db/TestOutOfMemory.java

@@ -37,6 +37,9 @@ public class TestOutOfMemory extends TestBase {
         if (config.memory || config.mvcc) {
             return;
         }
+        for (int i = 0; i < 5; i++) {
+            System.gc();
+        }
         deleteDb("outOfMemory");
         Connection conn = getConnection("outOfMemory");
         Statement stat = conn.createStatement();