Jackson による JSON シリアライズにおいて、JS の ``JSON.stringify()`` ぽい pretty print を実現する PrettyPrinter 実装です。

JsonStringifyPrettyPrinter.java

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.PrettyPrinter;

import java.io.IOException;
import java.util.Arrays;
import java.util.Stack;

/**
 * Google Chrome における JSON.stringify() に近い pretty print を実現します。
 * <p>
 * このオブジェクトはスレッドセーフではありません。
 * </p>
 *
 * @author KOMIYA Atsushi
 */
public class JsonStringifyPrettyPrinter implements PrettyPrinter {
    enum State {
        ROOT_VALUE_SEPARATOR,
        START_OBJECT,
        END_OBJECT,
        OBJECT_ENTRY_SEPARATOR,
        OBJECT_FIELD_VALUES_SEPARATOR,
        START_ARRAY,
        END_ARRAY,
        ARRAY_VALUE_SEPARATOR,
        BEFORE_ARRAY_VALUES,
        BEFORE_OBJECT_ENTRIES
    }

    private static final String LINE_SEPARATOR = System.lineSeparator();
    private static final char[] SPACES = new char[128];

    static {
        Arrays.fill(SPACES, ' ');
    }

    private final int numSpacesPerIndent;
    private int indentLevel;
    private State lastState;
    private Stack<Boolean> listContainsLiteralOnly = new Stack<>();

    /**
     * インデントのときのスペース個数を指定して、JsonStringifyPrettyPrinter オブジェクトを生成します。
     *
     * @param numSpacesPerIndent インデントのときのスペース個数を指定します
     */
    public JsonStringifyPrettyPrinter(int numSpacesPerIndent) {
        this.numSpacesPerIndent = numSpacesPerIndent;
    }

    private void indent(JsonGenerator jg) throws IOException {
        jg.writeRaw(LINE_SEPARATOR);

        int numSpacesToBeOutput = indentLevel * numSpacesPerIndent;
        while (numSpacesToBeOutput > SPACES.length) {
            jg.writeRaw(SPACES, 0, SPACES.length);
            numSpacesToBeOutput -= SPACES.length;
        }

        jg.writeRaw(SPACES, 0, numSpacesToBeOutput);
    }

    @Override
    public void writeRootValueSeparator(JsonGenerator jg) throws IOException {
        lastState = State.ROOT_VALUE_SEPARATOR;
    }

    @Override
    public void writeStartObject(JsonGenerator jg) throws IOException {
        if (lastState != State.OBJECT_FIELD_VALUES_SEPARATOR) {
            indent(jg);
        }
        jg.writeRaw("{");
        indentLevel++;

        if (!listContainsLiteralOnly.empty() && listContainsLiteralOnly.peek()) {
            listContainsLiteralOnly.pop();
            listContainsLiteralOnly.push(false);
        }

        lastState = State.START_OBJECT;
    }

    @Override
    public void writeEndObject(JsonGenerator jg, int nrOfEntries) throws IOException {
        indentLevel--;
        indent(jg);
        jg.writeRaw("}");

        lastState = State.END_OBJECT;
    }

    @Override
    public void writeObjectEntrySeparator(JsonGenerator jg) throws IOException {
        jg.writeRaw(",");
        indent(jg);

        lastState = State.OBJECT_ENTRY_SEPARATOR;
    }

    @Override
    public void writeObjectFieldValueSeparator(JsonGenerator jg) throws IOException {
        jg.writeRaw(": ");

        lastState = State.OBJECT_FIELD_VALUES_SEPARATOR;
    }

    @Override
    public void writeStartArray(JsonGenerator jg) throws IOException {
        if (lastState != State.OBJECT_FIELD_VALUES_SEPARATOR) {
            indent(jg);
        }
        jg.writeRaw("[");
        indentLevel++;

        listContainsLiteralOnly.push(true);

        lastState = State.START_ARRAY;
    }

    @Override
    public void writeEndArray(JsonGenerator jg, int nrOfValues) throws IOException {
        indentLevel--;

        if (!listContainsLiteralOnly.pop()) {
            indent(jg);
        }
        jg.writeRaw("]");

        lastState = State.END_ARRAY;
    }

    @Override
    public void writeArrayValueSeparator(JsonGenerator jg) throws IOException {
        jg.writeRaw(", ");

        lastState = State.ARRAY_VALUE_SEPARATOR;
    }

    @Override
    public void beforeArrayValues(JsonGenerator jg) throws IOException {
        lastState = State.BEFORE_ARRAY_VALUES;
    }

    @Override
    public void beforeObjectEntries(JsonGenerator jg) throws IOException {
        indent(jg);
        lastState = State.BEFORE_OBJECT_ENTRIES;
    }
}

JsonStringifyPrettyPrinterDemo.java

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.*;

/**
 * JsonStringifyPrettyPrinter 使い方デモ。
 */
public class JsonStringifyPrettyPrinterDemo {
    public static void main(String[] args) throws JsonProcessingException {
        List<Map<String, Object>> object = new ArrayList<>();

        Map<String, Object> map1 = new HashMap<>();
        map1.put("str", "hogefuga");
        map1.put("int", 123);
        map1.put("list", Arrays.asList(1, 1, 2, 3, 5));
        object.add(map1);

        Map<String, Object> map2 = new HashMap<>();
        map2.put("bool", true);
        map2.put("null", null);
        object.add(map2);

        // ObjectMapper#writer() で、PrettyPrinter オブジェクトを設定してあげる
        String json = new ObjectMapper()
                .writer(new JsonStringifyPrettyPrinter(2))
                .writeValueAsString(object);

        System.out.println(json);
    }
}

output.txt

[
  {
    "str": "hogefuga",
    "list": [1, 1, 2, 3, 5],
    "int": 123
  }, 
  {
    "bool": true,
    "null": null
  }
]