Introduction to APON (Aspectran Parameters Object Notation)

1. What is APON?

APON (Aspectran Parameters Object Notation) is a lightweight data-interchange format for representing structured data. While it has a structure similar to JSON, it is characterized by enhanced readability, making it easy for humans to read and write, much like YAML.

APON is specifically designed to simplify the creation of configuration files for the Aspectran framework and to ensure that applications can accurately read configuration values.

Key Features

• Excellent Readability: Items are separated by newlines instead of commas (,), and quotes can be omitted if the value contains no spaces or special characters, resulting in concise and clear code.
• Explicit Type Support: You can explicitly specify the data type for a value, ensuring the accuracy and stability of configuration values.
• Hierarchical Data Structure: Braces ({ }) can be used to structure data hierarchically, allowing for the systematic representation of complex configurations.
• Long String Support: Supports a text type that makes it easy to write multi-line text.
• Comment Support: The # character can be used to add comments to the code. It must be written on a new line.

2. Basic Syntax

Single Value (Parameter)

The most basic unit in APON can have a single value in the format name: value. It is common to omit quotes (") unless the value has the following special conditions:

• If there are spaces at the beginning or end of the value.
• If the value contains single (') or double (") quotes.
• If the value contains a newline character (\n).

# Example without double quotes
name: John Doe
age: 30
city: New York

# Example requiring double quotes
message: "'Hello, World'"
message: "First line\nNew line"
indented: "  Leading spaces"
indented: "Trailing spaces  "

Set (Parameters)

A set of Parameters is represented by enclosing them in braces ({ }). It can have a hierarchical structure containing other sub-Parameters.

user: {
  name: John Doe
  age: 30
  address: {
    city: New York
    zipCode: 10001
  }
}

Array

An array is represented by putting multiple values inside square brackets ([ ]). Each element is separated by a newline.

users: [
  John
  Jane
  Peter
]
numbers: [
  1
  2
  3
]

3. Data Types

APON supports various data types, and the type can be specified in the format name(type): value. If the type is omitted, it is automatically determined based on the value’s format.

text Type Usage Example

For long strings that span multiple lines, using the text type is convenient. It is enclosed in parentheses (( )), and each line is prefixed with the | character.

description(text): (
  |This is a multi-line text.
  |Each line starts with a pipe character.
  |Within this block, special characters like
  |"double quotes" or 'single quotes' can be used freely.
)

4. Using APON

Let’s explore how to convert APON-formatted text into a Java object and vice versa.

Step 1: Define Java Parameters Classes to Map to APON

You need to define classes in Java that implement the Parameters interface corresponding to the APON data structure. Each class corresponds to a hierarchical level in APON, and you define the name, type, and array status of each parameter using ParameterDefinition.

Example APON:

server: {
  name: MyServer
  port: 8080
  features: [
    HTTP2
    SSL
  ]
}

Java classes to map to the above APON:

RootConfig.java

import com.aspectran.utils.apon.AbstractParameters;
import com.aspectran.utils.apon.ParameterDefinition;
import com.aspectran.utils.apon.ParameterValueType;
import com.aspectran.utils.apon.Parameters;

public class RootConfig extends AbstractParameters implements Parameters {

    public static final ParameterDefinition server;

    private static final ParameterDefinition[] parameterDefinitions;

    static {
        // Has a hierarchical structure containing other Parameters
        server = new ParameterDefinition("server", ServerConfig.class);

        parameterDefinitions = new ParameterDefinition[] {
                server
        };
    }

    public RootConfig() {
        super(parameterDefinitions);
    }
}

ServerConfig.java

import com.aspectran.utils.apon.AbstractParameters;
import com.aspectran.utils.apon.ParameterDefinition;
import com.aspectran.utils.apon.ParameterValueType;
import com.aspectran.utils.apon.Parameters;

public class ServerConfig extends AbstractParameters implements Parameters {

    public static final ParameterDefinition name;
    public static final ParameterDefinition port;
    public static final ParameterDefinition features;

    private static final ParameterDefinition[] parameterDefinitions;

    static {
        name = new ParameterDefinition("name", ParameterValueType.STRING);
        port = new ParameterDefinition("port", ParameterValueType.INT);
        // The 'features' parameter is of string type and is an array that can hold multiple values (third argument is true)
        features = new ParameterDefinition("features", ParameterValueType.STRING, true);

        parameterDefinitions = new ParameterDefinition[] {
                name,
                port,
                features
        };
    }

    public ServerConfig() {
        super(parameterDefinitions);
    }
}

Step 2: Reading an APON File (AponReader)

The AponReader class allows you to read an APON-formatted text file and convert it into a defined Parameters Java object.

import com.aspectran.utils.apon.AponReader;
import com.aspectran.utils.apon.Parameters;
import java.io.FileReader;
import java.io.Reader;

public class AponTest {
    public static void main(String[] args) {
        try {
            // 1. Create a Reader to read the APON file
            Reader fileReader = new FileReader("root-config.apon");

            // 2. Create a top-level Parameters object defined according to the APON structure
            Parameters rootParams = new RootConfig(); // RootConfig contains the server parameter

            // 3. Use AponReader to read the file content into the Parameters object
            AponReader.read(fileReader, rootParams);

            // 4. Use the converted object
            Parameters serverConfig = rootParams.getParameters("server");
            String serverName = serverConfig.getString("name");
            int port = serverConfig.getInt("port");

            System.out.println("Server Name: " + serverName);
            System.out.println("Port: " + port);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Step 3: Creating an APON File (AponWriter)

The AponWriter class makes it easy to convert a Parameters Java object into an APON-formatted string.

AponWriter works intelligently, automatically enclosing values in double quotes and performing necessary escaping when quotes are required (e.g., if the value has leading/trailing spaces, or contains quotes or newline characters). Therefore, developers only need to set the values of the Parameters object without worrying about the quoting rules.

import com.aspectran.utils.apon.AponWriter;
import com.aspectran.utils.apon.Parameters;
import java.io.StringWriter;
import java.io.Writer;

public class AponWriteTest {
    public static void main(String[] args) {
        try {
            // 1. Create and set values for the Parameters object to be converted to APON
            Parameters serverConfig = new ServerConfig();
            serverConfig.putValue("name", "NewServer");
            serverConfig.putValue("port", 9090);
            serverConfig.putValue("features", new String[] {"WebService", "Security"});

            Parameters rootParams = new RootConfig(); // RootConfig contains the server parameter
            rootParams.putValue("server", serverConfig);

            // 2. Create a Writer to store the APON text
            Writer writer = new StringWriter();

            // 3. Use AponWriter to write the Parameters object in APON format
            try (AponWriterCloseable aponWriter = new AponWriterCloseable(writer)) {
                aponWriter.write(rootParams);
            }

            // 4. Print the converted APON string
            System.out.println(writer);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output Result:

server: {
  name: NewServer
  port: 9090
  features: [
    WebService
    Security
  ]
}

5. Library Information

APON-related classes are included in the common utility package of the Aspectran framework.

• Package Path: com.aspectran.core.util.apon
• Source Code: View on GitHub