feat: support -h\--help option (#21)

src/main/java/org/casbin/Client.java

@@ -20,6 +20,10 @@ public static String run(String... args) {
             }
 
             String commandName = args[0];
+            if(Objects.equals(commandName, "-h") || Objects.equals(commandName, "--help")){
+                printHelpMessage();
+                return result;
+            }
 
             CommandLine cmd = getCmd(Arrays.copyOfRange(args, 1, args.length));
             String model = cmd.getOptionValue("model");
@@ -40,6 +44,7 @@ public static String run(String... args) {
 
         } catch (Exception e) {
             e.printStackTrace();
+            System.out.println("Run './casbin --help or ./casbin -h' for usage.");
             System.exit(1);
         }
         return result;
@@ -77,4 +82,36 @@ private static CommandLine getCmd(String[] args) throws ParseException {
         CommandLineParser parser = new DefaultParser();
         return parser.parse(options, args);
     }
+
+    private static void printHelpMessage() {
+        System.out.println(" Usage: ./casbin [Method] [options] [args]\n" +
+                "\n" +
+                "    Casbin is a powerful and efficient open-source access control library.\n" +
+                "    It provides support for enforcing authorization based on various access control models.\n" +
+                "\n" +
+                "    Method:\n" +
+                "      enforce       Test if a 'subject' can access an 'object' with a given 'action' based on the policy\n" +
+                "      enforceEx     Check permissions and get which policy it matches\n" +
+                "      addFunction   Add custom function\n" +
+                "      addPolicy     Add a policy rule to the policy file\n" +
+                "      removePolicy  Remove a policy rule from the policy file\n" +
+                "    For more method, visit https://github.com/casbin/jcasbin\n" +
+                "\n" +
+                "    Options:\n" +
+                "      -m, --model <model>          The path of the model file or model text. Please wrap it with \"\" and separate each line with \"|\"\n" +
+                "      -p, --policy <policy>        The path of the policy file or policy text. Please wrap it with \"\" and separate each line with \"|\"\n" +
+                "      -AF, --addFunction <functionDefinition>       Add custom function. Please wrap it with \"\" and separate each line with \"|\"\n" +
+                "\n" +
+                "    args:\n" +
+                "      Parameters required for the method\n" +
+                "\n" +
+                "    Examples:\n" +
+                "      ./casbin enforce -m \"examples/rbac_model.conf\" -p \"examples/rbac_policy.csv\" \"alice\" \"data1\" \"read\"\n" +
+                "      ./casbin enforceEx -m \"examples/rbac_model.conf\" -p \"examples/rbac_policy.csv\" \"alice\" \"data1\" \"read\"\n" +
+                "      ./casbin addPolicy -m \"examples/rbac_model.conf\" -p \"examples/rbac_policy.csv\" \"alice\" \"data2\" \"write\"\n" +
+                "      ./casbin enforce -m  \"your_model.conf\" -p \"examples/keymatch_policy.csv\" -AF \"yourFunctionDefinition\" \"alice\" \"/alice_data/resource1\" \"GET\"\n" +
+                "\n" +
+                "    For more information, visit https://github.com/casbin/casbin");
+
+    }
 }