I have following json as string:
{
"field1":"value1",
"field2":"value2",
"field3":{
"field31":"value31",
"field32":"value32"
},
"field4":{
"field41":"value41"
}
}
What is the best and the most modern way to get from this json just value from field41, so I would return "value41". I know that I can use JSONObject but I'm wondering weather we have any other better option?
Try JSONPath
String json = "...";
String field41 = JsonPath.read(json, "$.field4.field41");
You can test it here - https://jsonpath.herokuapp.com/
If you want to generate a real object out of it you can use Gson. You need to describe the class first. There are online json to Java objects converters out there. And then you can just call:
YourObject obj = new Gson().fromJson(json,YourObject.class);
System.out.println(obj.getField4().getField41());
And there you have it!
I have a question regarding best practices considering Java regular expressions/Strings manipulation.
I have a changing String template, let's say this time it looks like this:
/get/{id}/person
I have another String that matches this pattern eg.
/get/1234ewq/person
Keep in mind that the pattern could change anytime, slashes could disappear etc.
I would like to extract the difference between the two of them i.e. the result of the processing would be 1234ewq.
I know I could iterate over them char by char and compare, but, if it is possible, I wanted to find some smart approach to it with regular expressions.
What would be the best Java approach?
Thank you.
For you to answer your question with a regex approach I built a small example class which should hint you into a direction you could go with this (see below).
The problem with this approach is that you dynamically create a regular expression that depends on your template strings. This means that you have to somehow verify that your templates do not interfere with the regex compilation and matching process itself.
Also atm if you would use the same placeholder multiple times within a template the resulting HashMap only contains the value for the last placeholder mapping of that kind.
Normally this is the expected behaviour but this depends on your strategy of filling your templates.
For template processing in general you could have a look at the mustache library.
Also as Uli Sotschok mentioned, you probably would be better of with using something like google-diff-match-patch.
public class StringExtractionFromTemplate {
public static void main(String[] args) {
String template = "/get/{id}/person";
String filledTemplate = "/get/1234ewq/person";
System.out.println(diffTemplateInsertion(template, filledTemplate).get("id"));
}
private static HashMap<String, String> diffTemplateInsertion(String template, String filledTemplate){
//language=RegExp
String placeHolderPattern = "\\{(.+)}";
HashMap<String, String> templateTranslation = new HashMap<>();
String regexedTemplate = template.replaceAll(placeHolderPattern, "(.+)");
Pattern pattern = Pattern.compile(regexedTemplate);
Matcher templateMatcher = pattern.matcher(template);
Matcher filledTemplateMatcher = pattern.matcher(filledTemplate);
while (templateMatcher.find() && filledTemplateMatcher.find()) {
if(templateMatcher.groupCount() == filledTemplateMatcher.groupCount()){
for (int i = 1; i <= templateMatcher.groupCount(); i++) {
templateTranslation.put(
templateMatcher.group(i).replaceAll(placeHolderPattern,"$1"),
filledTemplateMatcher.group(i)
);
}
}
}
return templateTranslation;
}
}
Is there any String replacement mechanism in Java, where I can pass objects with a text, and it replaces the string as it occurs?
For example, the text is:
Hello ${user.name},
Welcome to ${site.name}.
The objects I have are user and site. I want to replace the strings given inside ${} with its equivalent values from the objects. This is same as we replace objects in a velocity template.
Use StringSubstitutor from Apache Commons Text.
Dependency import
Import the Apache commons text dependency using maven as bellow:
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-text</artifactId>
<version>1.10.0</version>
</dependency>
Example
Map<String, String> valuesMap = new HashMap<String, String>();
valuesMap.put("animal", "quick brown fox");
valuesMap.put("target", "lazy dog");
String templateString = "The ${animal} jumped over the ${target}.";
StringSubstitutor sub = new StringSubstitutor(valuesMap);
String resolvedString = sub.replace(templateString);
Take a look at the java.text.MessageFormat class, MessageFormat takes a set of objects, formats them, then inserts the formatted strings into the pattern at the appropriate places.
Object[] params = new Object[]{"hello", "!"};
String msg = MessageFormat.format("{0} world {1}", params);
My preferred way is String.format() because its a oneliner and doesn't require third party libraries:
String message = String.format("Hello! My name is %s, I'm %s.", name, age);
I use this regularly, e.g. in exception messages like:
throw new Exception(String.format("Unable to login with email: %s", email));
Hint: You can put in as many variables as you like because format() uses Varargs
I threw together a small test implementation of this. The basic idea is to call format and pass in the format string, and a map of objects, and the names that they have locally.
The output of the following is:
My dog is named fido, and Jane Doe owns him.
public class StringFormatter {
private static final String fieldStart = "\\$\\{";
private static final String fieldEnd = "\\}";
private static final String regex = fieldStart + "([^}]+)" + fieldEnd;
private static final Pattern pattern = Pattern.compile(regex);
public static String format(String format, Map<String, Object> objects) {
Matcher m = pattern.matcher(format);
String result = format;
while (m.find()) {
String[] found = m.group(1).split("\\.");
Object o = objects.get(found[0]);
Field f = o.getClass().getField(found[1]);
String newVal = f.get(o).toString();
result = result.replaceFirst(regex, newVal);
}
return result;
}
static class Dog {
public String name;
public String owner;
public String gender;
}
public static void main(String[] args) {
Dog d = new Dog();
d.name = "fido";
d.owner = "Jane Doe";
d.gender = "him";
Map<String, Object> map = new HashMap<String, Object>();
map.put("d", d);
System.out.println(
StringFormatter.format(
"My dog is named ${d.name}, and ${d.owner} owns ${d.gender}.",
map));
}
}
Note: This doesn't compile due to unhandled exceptions. But it makes the code much easier to read.
Also, I don't like that you have to construct the map yourself in the code, but I don't know how to get the names of the local variables programatically. The best way to do it, is to remember to put the object in the map as soon as you create it.
The following example produces the results that you want from your example:
public static void main(String[] args) {
Map<String, Object> map = new HashMap<String, Object>();
Site site = new Site();
map.put("site", site);
site.name = "StackOverflow.com";
User user = new User();
map.put("user", user);
user.name = "jjnguy";
System.out.println(
format("Hello ${user.name},\n\tWelcome to ${site.name}. ", map));
}
I should also mention that I have no idea what Velocity is, so I hope this answer is relevant.
Here's an outline of how you could go about doing this. It should be relatively straightforward to implement it as actual code.
Create a map of all the objects that will be referenced in the template.
Use a regular expression to find variable references in the template and replace them with their values (see step 3). The Matcher class will come in handy for find-and-replace.
Split the variable name at the dot. user.name would become user and name. Look up user in your map to get the object and use reflection to obtain the value of name from the object. Assuming your objects have standard getters, you will look for a method getName and invoke it.
There are a couple of Expression Language implementations out there that does this for you, could be preferable to using your own implementation as or if your requirments grow, see for example JUEL and MVEL
I like and have successfully used MVEL in at least one project.
Also see the Stackflow post JSTL/JSP EL (Expression Language) in a non JSP (standalone) context
Handlebars.java might be a better option in terms of a Velocity-like syntax with other server-side templating features.
http://jknack.github.io/handlebars.java/
Handlebars handlebars = new Handlebars();
Template template = handlebars.compileInline("Hello {{this}}!");
System.out.println(template.apply("Handlebars.java"));
I use GroovyShell in java to parse template with Groovy GString:
Binding binding = new Binding();
GroovyShell gs = new GroovyShell(binding);
// this JSONObject can also be replaced by any Java Object
JSONObject obj = new JSONObject();
obj.put("key", "value");
binding.setProperty("obj", obj)
String str = "${obj.key}";
String exp = String.format("\"%s\".toString()", str);
String res = (String) gs.evaluate(exp);
// value
System.out.println(str);
I created this utility that uses vanilla Java. It combines two formats... {} and %s style from String.format.... into one method call. Please note it only replaces empty {} brackets, not {someWord}.
public class LogUtils {
public static String populate(String log, Object... objects) {
log = log.replaceAll("\\{\\}", "%s");
return String.format(log, objects);
}
public static void main(String[] args) {
System.out.println(populate("x = %s, y ={}", 5, 4));;
}
}
Since Java 15 you have the method String.formatted() (see documentation).
str.formatted(args) is the equivalent of String.format(str, args) with less ceremony.
For the example mentioned in the question, the method could be used as follows:
"Hello %s, Welcome to %s.".formatted(user.getName(), site.getName())
Good news. Java is most likely going to have string templates (probably from version 21).
See the string templates proposal (JEP 430) here.
It will be something along the lines of this:
String name = "John";
String info = STR."I am \{name}";
System.out.println(info); // I am John
P.S. Kotlin is 100% interoperable with Java. It supports cleaner string templates out of the box:
val name = "John"
val info = "I am $name"
println(info) // I am John
Combined with extension functions, you can achieve the same thing the Java template processors (e.g. STR) will do.
There is nothing out of the box that is comparable to velocity since velocity was written to solve exactly that problem. The closest thing you can try is looking into the Formatter
http://cupi2.uniandes.edu.co/site/images/recursos/javadoc/j2se/1.5.0/docs/api/java/util/Formatter.html
However the formatter as far as I know was created to provide C like formatting options in Java so it may not scratch exactly your itch but you are welcome to try :).
I have the following class. I use ObjectMapper.convertValue() to convert this class to Jackson ObjectNode. Then ObjectNode.toString() will return a String like "{"playing": false}".
But what I am looking for is the Json String without escaping quote like "{\"playing\": false}".
I am currently using String.replace("\"", "\\\""), and it worked. Is there any better way to achieve that?
Update Node: As mentioned in the comment, I need to send this state String to server, but it looks like my server can only recognize "{\"playing\": false}".
public class MyState {
public Boolean playing;
}
private String getStateString()
throws Exception
{
ObjectNode objectNode = objectMapper.convertValue(currentState, ObjectNode.class);
String pretty = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(objectNode);
return objectNode.toString().replace("\"", "\\\"");
}
Update Solution: As mentioned in comment, the root cause that the service was not able to process my state string is not due to string escape, but due to incorrect attribute name (which is further converted to json key String).
This question may have been answered before in some dark recess of the Interwebs, but I couldn't even figure out how to form a meaningful Google query to search for it.
So: Suppose I have a (simplified) XML document like so:
<root>
<tag1>Value</tag1>
<tag2>Word</tag2>
<tag3>
<something1>Foo</something1>
<something2>Bar</something2>
<something3>Baz</something3>
</tag3>
</root>
I know how to use JAXB to unmarshal this into a Java Object in the standard use cases.
What I don't know how to do is unmarshal tag3's contents wholesale into a String. By which I mean:
<something1>Foo</something1>
<something2>Bar</something2>
<something3>Baz</something3>
as a String, tags and all.
Use annotation #XmlAnyElement.
I've been looking for the same solution and I expected to find some annotation that prevents parsing dom and live it as it is, but did not find it.
Detail at:
Using JAXB to extract inner text of XML element
and
http://blog.bdoughan.com/2011/04/xmlanyelement-and-non-dom-properties.html
I added one cheking in method getElement(), otherwise we could get IndexOutOfBoundsException
if (xml.indexOf(START_TAG) < 0) {
return "";
}
For me it's quite strange behavior with this solution. method getElement() is called for every tag of your xml. The first call is for "Value", the second - "ValueWord", etc. It appends the next tag for previous
update:
I noticed that this approach works only for ONE occurence of tag that we want to parse to String. It's impossible to parse correctly the followint example:
<root>
<parent1>
<tag1>Value</tag1>
<tag2>Word</tag2>
<tag3>
<something1>Foo</something1>
<something2>Bar</something2>
<something3>Baz</something3>
</tag3>
</parent1>
<parent2>
<tag1>Value</tag1>
<tag2>Word</tag2>
<tag3>
<something1>TheSecondFoo</something1>
<something2>TheSecondBar</something2>
<something3>TheSecondBaz</something3>
</tag3>
</parent2>
"tag3" with parent tag "parent2" will contain parameters from the first tag (Foo, Bar, Baz) instead of (TheSecondFoo, TheSecondBar, TheSecondBaz)
Any suggestions are appreciated.
Thanks.
I have an utility method that might come in handy for you in that case. See if it helps. I made a sample code with your example:
public static void main(String[] args){
String text= "<root><tag1>Value</tag1><tag2>Word</tag2><tag3><something1>Foo</something1><something2>Bar</something2><something3>Baz</something3></tag3></root>";
System.out.println(extractTag(text, "<tag3>"));
}
public static String extractTag(String xml, String tag) {
String value = "";
String endTag = "</" + tag.substring(1);
Pattern p = Pattern.compile(tag + "(.*?)" + endTag);
Matcher m = p.matcher(xml);
if (m.find()) {
value = m.group(1);
}
return value;
}