在Jackson中将JsonNode序列化为非常特定的JSON格式

Question

我有JsonNode结果，我想打印出来。到目前为止，我使用的是：

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);

它输出类似以下内容：

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

但是我需要它是这个特定的格式：

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

对于上下文，我从JSONObject转换到了Jackson，所以第二个输出是由JSONObject.serialize()输出的。

另外，上面提到的每种格式都有一个名称吗？它似乎遵循不同的标准。

Answer 1

您可以自定义Jackson将如何缩进输出。根据你正在使用的Jackson版本，有不同的方法可以实现它。

Jackson 2.5及更新版本

执行以下操作：

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，请使用接收字符串以缩进级别的DefaultIndenter构造函数和行分隔符：

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

Jackson 2.4及更早版本

执行以下操作：

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);

Lf2SpacesIndenter限制为2个空格，您无法对其进行更改。

如果您需要不同数量的空格，则需要编写自定义实现。下面是一个与Jackson 2.5中引入的DefaultIndenter使用相同代码的示例：

/**
 * 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);
        }
    }
}

它的使用方法如下：

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

Answer 2

您可以使用以下命令设置自定义DefaultPrettyPrinter：

DefaultPrettyPrinter pp = new DefaultPrettyPrinter();
pp.indentObjectsWith(new Lf2SpacesIndenter());
pp.indentArraysWith(new Lf2SpacesIndenter("\r\n"));
mapper.writer(pp).writeValue(new FileOutputStream(outputFile), resultNode);

看看DefaultPrettyPrinter HERE提供的方法