在Jackson中将JsonNode序列化为非常特定的JSON格式

Question

我有JsonNode要打印的结果。到目前为止，我正在使用：

ObjectMapper mapper = new ObjectMapper();
mapper.enable(SerializationFeature.INDENT_OUTPUT);
File outputFile = new File(
    getCurOutputDir(), String.format("out.json", getClass().getSimpleName())
);
mapper.writeValue(new FileOutputStream(outputFile), resultNode);

输出类似：

{
  "A" : [ {
    "Ai" : {
      "Ai1" : 42,
      "Ai2" : 55
    }
  } ],
    "B" : [ 86 ]
}

但我需要采用以下特定格式：

{
  "A" : [ 
    {
      "Ai" : {
        "Ai1" : 42,
        "Ai2" : 55
      }
    } 
  ],
    "B" : [
      86 
    ]
}

对于上下文，我要从过渡JSONObject到Jackson，因此第二个输出是的输出JSONObject.serialize()。

另外，上面列出的每种格式都有名称吗？似乎它遵循不同的标准。

Answer 1

您可以自定义Jackson如何缩进输出。根据您使用的Jackson版本，有多种实现方法。

杰克逊2.5及更高版本

请执行下列操作：

DefaultPrettyPrinter printer = new DefaultPrettyPrinter();
Indenter indenter = new DefaultIndenter();
printer.indentObjectsWith(indenter); // Indent JSON objects
printer.indentArraysWith(indenter);  // Indent JSON arrays

ObjectMapper mapper = new ObjectMapper();
mapper.writer(printer).writeValue(new FileOutputStream(outputFile), node);

默认情况下，将使用2个空格。对于不同数量的空格，请使用DefaultIndenter接收字符串以缩进级别和行分隔符的构造函数：

Indenter indenter = new DefaultIndenter("      ", DefaultIndenter.SYS_LF);

杰克逊2.4及更低版本

请执行下列操作：

DefaultPrettyPrinter printer = new DefaultPrettyPrinter();
Indenter indenter = new Lf2SpacesIndenter();
printer.indentObjectsWith(indenter); // Indent JSON objects
printer.indentArraysWith(indenter);  // Indent JSON arrays

ObjectMapper mapper = new ObjectMapper();
mapper.writer(printer).writeValue(new FileOutputStream(outputFile), node);

将Lf2SpacesIndenter被限制在2个空格，你不能改变它。

如果需要不同数量的空格，则需要编写自定义实现。这是使用与DefaultIndenterJackson
2.5中引入的代码相同的代码的代码：

/**
 * Default linefeed-based indenter.
 */
public class CustomSpaceIndenter extends DefaultPrettyPrinter.NopIndenter {

    public final static String SYS_LF;
    static {
        String lf;
        try {
            lf = System.getProperty("line.separator");
        } catch (Throwable t) {
            lf = "\n"; // fallback when security manager denies access
        }
        SYS_LF = lf;
    }

    public static final CustomSpaceIndenter SYSTEM_LINEFEED_INSTANCE = 
            new CustomSpaceIndenter("  ", SYS_LF);

    /**
     * We expect to rarely get indentation deeper than this number of levels,
     * and try not to pre-generate more indentations than needed.
     */
    private final static int INDENT_LEVELS = 16;
    private final char[] indents;
    private final int charsPerLevel;
    private final String eol;

    /**
     * Indent with two spaces and the system's default line feed
     */
    public CustomSpaceIndenter() {
        this("  ", SYS_LF);
    }

    /**
     * Create an indenter which uses the <code>indent</code> string to indent one level
     *  and the <code>eol</code> string to separate lines.
     */
    public CustomSpaceIndenter(String indent, String eol)  {
        charsPerLevel = indent.length();
        indents = new char[indent.length() * INDENT_LEVELS];
        int offset = 0;
        for (int i=0; i<INDENT_LEVELS; i++) {
            indent.getChars(0, indent.length(), indents, offset);
            offset += indent.length();
        }
        this.eol = eol;
    }

    public CustomSpaceIndenter withLinefeed(String lf) {
        if (lf.equals(eol)) {
            return this;
        }
        return new CustomSpaceIndenter(getIndent(), lf);
    }

    public CustomSpaceIndenter withIndent(String indent) {
        if (indent.equals(getIndent())) {
            return this;
        }
        return new CustomSpaceIndenter(indent, eol);
    }

    public String getEol() {
        return eol;
    }

    public String getIndent() {
        return new String(indents, 0, charsPerLevel);
    }

    @Override
    public boolean isInline() { 
        return false;
    }

    @Override
    public void writeIndentation(JsonGenerator jg, int level) throws IOException {
        jg.writeRaw(eol);
        if (level > 0) { // should we err on negative values (as there's some flaw?)
            level *= charsPerLevel;
            while (level > indents.length) { // unlike to happen but just in case
                jg.writeRaw(indents, 0, indents.length); 
                level -= indents.length;
            }
            jg.writeRaw(indents, 0, level);
        }
    }
}

可以如下使用：

Indenter indenter = new CustomSpaceIndenter("      ", CustomSpaceIndenter.SYS_LF);