How to Pretty-Print XML with Proper Indentation and Doctype Positioning Using Java's Standard API

How can I pretty-print XML output with correct indentation and doctype positioning using only the standard Java API?

package test;

import java.io.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;

public class TestOutputKeys {
    public static void main(String[] args) throws TransformerException {
        Source xmlInput = new StreamSource(new StringReader(
                "<!-- Document comment --><aaa><bbb/><ccc/></aaa>"));
        StreamResult xmlOutput = new StreamResult(new StringWriter());

        Transformer transformer = TransformerFactory.newInstance().newTransformer();
        transformer.setOutputProperty(OutputKeys.DOCTYPE_SYSTEM, "testing.dtd");
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty("indent-amount", "4"); // Set indentation level
        transformer.transform(xmlInput, xmlOutput);

        System.out.println(xmlOutput.getWriter().toString());
    }
}

To pretty-print XML using the standard Java API while ensuring correct indentation and proper doctype positioning, adjustments must be made in the Transformer properties. Here's a detailed process to achieve the desired output.

transformer.setOutputProperty("indent-amount", "4"); // Configure indentation level.

Causes

• The doctype declaration often appears after the comment section due to how the Transformer processes XML. Unfortunately, there is no direct property to control doctype positioning.
• By default, indentation may not function as expected due to missing configuration parameters.

Solutions

• Set the `indent-amount` property to define the number of spaces used for indentation. It controls the visual structure of the XML output.
• To manage the doctype positioning, certain custom solutions or manipulations are necessary, as the standard API does not allow for explicit control over this aspect.

Mistake: Failing to set the `indent-amount` property, leading to no visible indentation in the output.

Solution: Always specify the `indent-amount` property after enabling indentation.

Mistake: Expecting the doctype to be placed before comments without additional configuration or filtering.

Solution: Consider post-processing your XML string to rearrange the doctype declaration if standard API features do not support it.