How to design REST URI for multiple Key-Value params of HTTP GET

Here is one idea to pass a parameter:

/products?productDetail=[{"key":"key0","value":"key1"},{"key":"key2","value":"key2"},{"key":"key3","value":"key3"}]

where

[{"key":"key0","value":"key1"},{"key":"key2","value":"key2"},{"key":"key3","value":"key3"}]

is a JSON representation of the List<kv> class

class kv {
    String key;
    String value;


    public kv(String key, String value) {
        super();
        this.key = key;
        this.value = value;
    }

    public String getKey() {
        return key;
    }

    public void setKey(String key) {
        this.key = key;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

}

so you can easily convert query parameter productDetail in to List<kv> using

new Gson().fromJson(productDetail,kv.class);

than you can easily iterate all elements.

Another suggestion is, if you don't know how many products are queried then use a POST request for this.

I would expand upon your second suggestion a little by adding explicit names for the parts of your product, and using semicolons in place of commas to separate product attributes, since the order does not matter^*.

/products?id=1;qty=44&qty=55;id=2

Note how id and qty are switched around for the second product, because the order of attributes does not matter.

---

^* There is a convention to use commas when the order is important, and semicolons when the order is not important.