{"id":995,"date":"2020-04-07T20:38:37","date_gmt":"2020-04-07T20:38:37","guid":{"rendered":"https:\/\/jan.schnasse.org\/blog\/?p=995"},"modified":"2026-09-28T11:38:46","modified_gmt":"2026-09-28T09:38:46","slug":"java-simple-serializing-of-maps-as-json","status":"publish","type":"post","link":"https:\/\/jan.schnasse.org\/blog\/2020\/04\/07\/java-simple-serializing-of-maps-as-json\/","title":{"rendered":"Java: Simple Read\/Write of Maps as Json"},"content":{"rendered":"
\n

You can use tools like Jackson<\/a> to serialize and deserialize maps to json an vice versa.<\/p>\n

Example<\/strong><\/p>\n

@Test<\/span>\npublic<\/span> void<\/span> loadMapFromFileAndSaveIt<\/span>(){<\/span>\n    Map<\/span><<\/span>Object<\/span>,<\/span> Object<\/span>><\/span> map <\/span>=<\/span> loadMap<\/span>(<\/span>\"map.json\"<\/span>);<\/span>\n    map<\/span>.<\/span>put<\/span>(<\/span>\"8\"<\/span>,<\/span> \"8th\"<\/span>);<\/span>\n    map<\/span>.<\/span>remove<\/span>(<\/span>\"7\"<\/span>);<\/span>\n    saveMap<\/span>(<\/span>map<\/span>,<\/span>\"\/path\/to\/map2.txt\"<\/span>);<\/span>\n}<\/span>\n\nprivate<\/span> Map<\/span><<\/span>Object<\/span>,<\/span> Object<\/span>><\/span> loadMap<\/span>(<\/span>String<\/span> string<\/span>)<\/span> {<\/span>\n    ObjectMapper<\/span> mapper <\/span>=<\/span> new<\/span> ObjectMapper<\/span>(); \/\/should be initialized outside!<\/span>\n    try<\/span> (<\/span>InputStream<\/span> in <\/span>=<\/span> Thread<\/span>.<\/span>currentThread<\/span>().<\/span>getContextClassLoader<\/span>().<\/span>getResourceAsStream<\/span>(<\/span>\"map.json\"<\/span>))<\/span> {<\/span>\n        return<\/span> mapper<\/span>.<\/span>readValue<\/span>(<\/span>in<\/span>,<\/span> HashMap<\/span>.<\/span>class<\/span>);<\/span>\n    }<\/span>catch<\/span> (<\/span>Exception<\/span> e<\/span>)<\/span> {<\/span>\n        throw<\/span> new<\/span> RuntimeException<\/span>(<\/span>e<\/span>);<\/span>\n    }<\/span>\n}<\/span>\n\nprivate<\/span> void<\/span> saveMap<\/span>(<\/span>Map<\/span><<\/span>Object<\/span>,<\/span> Object<\/span>><\/span> map<\/span>,<\/span>String<\/span> path<\/span>)<\/span> {<\/span>\n    try<\/span> (<\/span>PrintWriter<\/span> out <\/span>=<\/span> new<\/span> PrintWriter<\/span>(<\/span>path<\/span>))<\/span> {<\/span>\n        out<\/span>.<\/span>println<\/span>(<\/span>toString<\/span>(<\/span>map<\/span>));<\/span>\n    }<\/span> catch<\/span> (<\/span>Exception<\/span> e<\/span>)<\/span> {<\/span>\n        throw<\/span> new<\/span> RuntimeException<\/span>(<\/span>e<\/span>);<\/span>\n    }<\/span>\n}<\/span>\n\npublic<\/span> String<\/span> toString<\/span>(<\/span>Object<\/span> obj<\/span>)<\/span> {<\/span>\n    try<\/span> (<\/span>StringWriter<\/span> w <\/span>=<\/span> new<\/span> StringWriter<\/span>();)<\/span> {<\/span>\n        new<\/span> ObjectMapper<\/span>().<\/span>configure<\/span>(<\/span>SerializationFeature<\/span>.<\/span>INDENT_OUTPUT<\/span>,<\/span> true<\/span>).<\/span>writeValue<\/span>(<\/span>w<\/span>,<\/span> obj<\/span>);<\/span>\n        return<\/span> w<\/span>.<\/span>toString<\/span>();<\/span>\n    }<\/span> catch<\/span> (<\/span>Exception<\/span> e<\/span>)<\/span> {<\/span>\n        throw<\/span> new<\/span> RuntimeException<\/span>(<\/span>e<\/span>);<\/span>\n    }<\/span>\n}<\/span><\/code><\/pre>\n
If the file map.json<\/code> on your classpath contains<\/p>\n
{<\/span>\n \"1\"<\/span>:<\/span>\"1th\"<\/span>,<\/span>\n \"2\"<\/span>:<\/span>\"2th\"<\/span>,<\/span>\n \"3\"<\/span>:<\/span>\"3th\"<\/span>,<\/span>\n \"4\"<\/span>:<\/span>\"4th\"<\/span>,<\/span>\n \"5\"<\/span>:<\/span>\"5th\"<\/span>,<\/span>\n \"6\"<\/span>:<\/span>\"6th\"<\/span>,<\/span>\n \"7\"<\/span>:<\/span>\"7th\"<\/span>\n}<\/span><\/code><\/pre>\n
The code above will modify it and writes it to a file \/path\/to\/map2.txt<\/code> that will contain the modified map.<\/p>\n
{<\/span>\n  \"1\"<\/span> :<\/span> \"1th\"<\/span>,<\/span>\n  \"2\"<\/span> :<\/span> \"2th\"<\/span>,<\/span>\n  \"3\"<\/span> :<\/span> \"3th\"<\/span>,<\/span>\n  \"4\"<\/span> :<\/span> \"4th\"<\/span>,<\/span>\n  \"5\"<\/span> :<\/span> \"5th\"<\/span>,<\/span>\n  \"6\"<\/span> :<\/span> \"6th\"<\/span>,<\/span>\n  \"8\"<\/span> :<\/span> \"8th\"<\/span>\n}<\/span><\/code><\/pre>\n<\/div>\n
\n
 <\/p>\n
\n
<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
You can use tools like Jackson to serialize and deserialize maps to json an vice versa. Example @Test public void loadMapFromFileAndSaveIt(){ Map<Object, Object> map = loadMap(„map.json“); map.put(„8“, „8th“); map.remove(„7″); saveMap(map,“\/path\/to\/map2.txt“); } private Map<Object, Object> loadMap(String string) { ObjectMapper mapper = new ObjectMapper(); \/\/should be initialized outside! try (InputStream in = Thread.currentThread().getContextClassLoader().getResourceAsStream(„map.json“)) { return mapper.readValue(in, HashMap.class); […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6,9],"tags":[],"class_list":["post-995","post","type-post","status-publish","format-standard","hentry","category-development","category-java"],"_links":{"self":[{"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/posts\/995","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/comments?post=995"}],"version-history":[{"count":1,"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/posts\/995\/revisions"}],"predecessor-version":[{"id":4199,"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/posts\/995\/revisions\/4199"}],"wp:attachment":[{"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/media?parent=995"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/categories?post=995"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/jan.schnasse.org\/blog\/wp-json\/wp\/v2\/tags?post=995"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}