Implement visitor pattern for JSON traversal

[glassfishrobot]

Implement visitor pattern for easier and more convenient JSON traversal/introspection

Here is a first sketch:

New method for JsonValue to make it visitable

public void accept(JsonValueVisitor visitor);

Give the json constants a type

package javax.json;

//tagging interface for null, true, false public interface JsonConstant extends JsonValue{

}

And finally the visitor interface

package javax.json;

public interface JsonValueVisitor {

    public void visit(JsonArray array);
    public void visit(JsonObject object);
    public void visit(JsonNumber number);
    public void visit(JsonString string);
    public void visit(JsonConstant constant); //true, false, null 
}

[glassfishrobot]

Reported by salyh

[glassfishrobot]

kchung said: I've actually implemented a Visitor for JSON. It is similar to what's proposed here. The main differences are:

1. Mine has visitors for array elements and name/value pairs
2. Minie has no need for a JsonConstant, since constants are part of the above

For future reference, here's what I have

**Visitor.java**package javax.json;

import java.util.Map;

public class Visitor {

    public void visit(String key, JsonObject obj) {
        visit(obj);
    }

    public void visit(String key, JsonArray array) {
        visit(array);
    }

    public void visit(String key, String string) {
    }

    public void visit(String key, JsonNumber number) {
    }

    public void visit(String key, boolean test) {
    }

    public void visit(String key) {
    }

    public void visit(int i, JsonObject obj) {
        visit(obj);
    }

    public void visit(int i, JsonArray array) {
        visit(array);
    }

    public void visit(int i, String string) {
    }

    public void visit(int i, JsonNumber number) {
    }

    public void visit(int i, boolean test) {
    }

    public void visit(int i) {
    }

    public void visit(JsonObject object) {
        for (Map.Entry<String, JsonValue> entry: object.entrySet()) {
            String key = entry.getKey();
            JsonValue value = entry.getValue();
            switch (value.getValueType()) {
case OBJECT:
    visit(key, (JsonObject)value);
    break;
case ARRAY:
    visit(key, (JsonArray)value);
    break;
case NUMBER:
    visit(key, object.getJsonNumber(key));
    break;
case TRUE:
case FALSE:
    visit(key, object.getBoolean(key));
    break;
case STRING:
    visit(key, object.getString(key));
    break;
case NULL:
    visit(key);
    break;
            }
        }
    }

    public void visit(JsonArray array) {
        for (int i = 0; i < array.size(); i++) {
            switch (array.get(i).getValueType()) {
case OBJECT:
    visit(i, array.getJsonObject(i));
    break;
case ARRAY:
    visit(i, array.getJsonArray(i));
    break;
case NUMBER:
    visit(i, array.getJsonNumber(i));
    break;
case TRUE:
case FALSE:
    visit(i, array.getBoolean(i));
    break;
case STRING:
    visit(i, array.getString(i));
    break;
case NULL:
    visit(i);
    break;
             }
        }
    }
}

[glassfishrobot]

kchung said: However, I don't think visitors are useful. They are most useful in cases where context is not needed, like output the words in a dictionary, or compute the average age of all people. It becomes awkward when context is needed, like computing the average age of males who live in California. I have yet to come with an example where the use of visitors is compelling.

I think it would be more useful to look into using JDK's stream operations for JSON queries.

[glassfishrobot]

salyh said: Do we want to discuss JDK's streams within JSR 374?

[glassfishrobot]

kchung said: Yes, we'll look into applying JDK's streams on JSON objects. I'll open another jira to discuss it.

[glassfishrobot]

This issue was imported from java.net JIRA JSON_PROCESSING_SPEC-64

[glassfishrobot]

Marked as won't fix on Tuesday, March 7th 2017, 9:07:29 am