Try this:
ObjectMapper mapper = new ObjectMapper();
ObjectNode rootNode = mapper.createObjectNode();
ObjectNode childNode1 = mapper.createObjectNode();
childNode1.put("name1", "val1");
childNode1.put("name2", "val2");
rootNode.set("obj1", childNode1);
ObjectNode childNode2 = mapper.createObjectNode();
childNode2.put("name3", "val3");
childNode2.put("name4", "val4");
rootNode.set("obj2", childNode2);
ObjectNode childNode3 = mapper.createObjectNode();
childNode3.put("name5", "val5");
childNode3.put("name6", "val6");
rootNode.set("obj3", childNode3);
String jsonString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(rootNode);
System.out.println(jsonString);
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