performance improvement:

public void func() {
   synchronized(this) {
       ...
   }
}

is less efficient than

public synchronized void func() {
   ...
}

src/debug/org/luaj/debug/DebugLuaState.java

@@ -483,46 +483,39 @@ public class DebugLuaState extends LuaState implements DebugRequestListener {
     /**
      * suspend the execution
      */
-    public void suspend() {
+    public synchronized void suspend() {
-        synchronized (this) {
         suspended = true;
         stepping = STEP_NONE;
         lastline = -1;
         this.notify();
     }
-    }
 
     /**
      * If the VM is suspended on start, this method resumes the execution.
      */
-    protected void cancelSuspendOnStart() {
+    protected synchronized void cancelSuspendOnStart() {
-        synchronized (this) {
         if (bSuspendOnStart) {
             bSuspendOnStart = false;
             this.notify();
         }
     }
-    }
 
     /**
      * resume the execution
      */
-    public void resume() {
+    public synchronized void resume() {
-        synchronized (this) {
         this.suspended = false;
         this.stepping = STEP_NONE;
         this.shouldPauseForStepping = false;
         this.steppingFrame = -1;
         this.notify();
     }
-    }
 
     /**
      * Stops the debugging communication with the debug client and terminate the
      * VM execution.
      */
-    public void stop() {
+    public synchronized void stop() {
-        synchronized (this) {
         if (exiting) return;
         
         if (this.debugSupport != null) {
@@ -537,7 +530,6 @@ public class DebugLuaState extends LuaState implements DebugRequestListener {
             debugSupport = null;       
         }
     }
-    }
 
     public void disconnect() {
         if (this.debugSupport == null) {
@@ -561,24 +553,20 @@ public class DebugLuaState extends LuaState implements DebugRequestListener {
         debugSupport.disconnect();                           
     }
     
-    public void reset() {
+    public synchronized void reset() {
-        synchronized (this) {
         this.breakpoints.clear();
         if (this.suspended) {
             resume();                
         }
     }
-    }
     
     /**
      * terminate the execution
      */
-    public void exit() {
+    public synchronized void exit() {
-        synchronized (this) {
         exiting = true;
         this.notify();
     }
-    }
 
     /**
      * set breakpoint at line N
@@ -822,35 +810,29 @@ public class DebugLuaState extends LuaState implements DebugRequestListener {
     /**
      * step over to next line
      */
-    public void stepOver() {
+    public synchronized void stepOver() {
-        synchronized (this) {
         suspended = false;
         stepping = STEP_OVER;
         steppingFrame = cc;
         this.notify();
     }
-    }
 
     /**
      * step to the next statement
      */
-    public void stepInto() {
+    public synchronized void stepInto() {
-        synchronized (this) {
         suspended = false;
         stepping = STEP_INTO;
         this.notify();
     }
-    }
 
     /**
      * return from the current method call
      */
-    public void stepReturn() {
+    public synchronized void stepReturn() {
-        synchronized (this) {
         suspended = false;
         stepping = STEP_RETURN;
         steppingFrame = cc;
         this.notify();
     }
 }
-}