Regarding JsonObject and LinkedHashMap implementation

[Yeikel]

Hi team, 

I just took a memory dump of my application and I noticed that there is a considerable waste of memory due to the LinkedHashMap that backs JsonObjects. 

From the public documentation : 

/**
* Constructs an empty insertion-ordered {@code LinkedHashMap} instance
* with the default initial capacity (16) and load factor (0.75).
*/
public LinkedHashMap() {
super();
accessOrder = false;
}

This usually leads to objects such as : 

LinkedHashMap(size: 2, capacity: 4) {(key:"email", val:"em...@example.com"), (key:"standard", val:"Y")}

j.u.LinkedHashMap<String, ?>(size: 3, capacity: 16) {(key:"xxx", val:j.u.LinkedHashMap(size : 2, modCount : 2, threshold : 3, ...)), (key:"id", val:"123"), (key:"conf_cd", val:"LOW")}

I understand that this is how Java collection works, but is there anything we can do? 

I was thinking that perhaps we could add a new constructor of JsonObject that allows configuring the  initialCapacity and loadFactor but I do not think that this will fix the problem permanently as there are many other places that we use JsonObject. 

Adding a default capacity in the original constructor may lead to considerable CPU waste due to the resizing needed. 

Similar issue in Hadoop : https://issues.apache.org/jira/browse/MAPREDUCE-7150 

Relevant article : https://dzone.com/articles/preventing-your-java-collections-from-wasting-memo 

[Thomas SEGISMONT]

There's no silver bullet for sizing, depends on the application.

To give you maximum flexibility there is a special constructor of JsonObject which takes a Map parameter: io.vertx.core.json.JsonObject#JsonObject(java.util.Map<java.lang.String,java.lang.Object>)

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