Add methods to get and set arrow knockback. Adds BUKKIT-5103

src/main/java/net/minecraft/server/EntityArrow.java

@@ -21,7 +21,7 @@ public class EntityArrow extends Entity implements IProjectile {
     private int j;
     private int au;
     private double damage = 2.0D;
-    private int aw;
+    public int aw; // CraftBukkit - private -> public
 
     public EntityArrow(World world) {
         super(world);

src/main/java/org/bukkit/craftbukkit/entity/CraftArrow.java

@@ -2,6 +2,7 @@ package org.bukkit.craftbukkit.entity;
 
 import net.minecraft.server.EntityArrow;
 
+import org.apache.commons.lang.Validate;
 import org.bukkit.craftbukkit.CraftServer;
 import org.bukkit.entity.Arrow;
 import org.bukkit.entity.EntityType;
@@ -27,6 +28,15 @@ public class CraftArrow extends AbstractProjectile implements Arrow {
         }
     }
 
+    public void setKnockbackStrength(int knockbackStrength) {
+        Validate.isTrue(knockbackStrength >= 0, "Knockback cannot be negative");
+        getHandle().a(knockbackStrength);
+    }
+
+    public int getKnockbackStrength() {
+        return getHandle().aw;
+    }
+
     @Override
     public EntityArrow getHandle() {
         return (EntityArrow) entity;