Develop Reference

How to wait for a JavaFX Service to finish before returning data in caller?

Working on my first Java project I can't seem to get around this probably basic problem: In a JavaFX application I have a DAO class, which starts a service to get values from a mysql db, builds an object from it and returns the object to the caller. But the object never gets build, because the return happens before the service has succeeded.
public IQA getQA(int id) throws SQLException {
try {
GetQuizService getQuizService = new GetQuizService();
getQuizService.restart();
getQuizService.setId(id);
getQuizService.setOnSucceeded(e -> {
this.quiz = getQuizService.getValue();
});
} catch (Exception e) {
System.err.println(e);
}
return quiz;
}
The service works fine, inside the onSucceeded action the object is present, but how can I make the return wait until the service has finished?
As requested here's a minimal version of the GetQuizService
public class GetQuizService extends Service<Quiz> {
private int id;
private Quiz quiz;
public void setId(int id) {
this.id = id;
}
#Override
protected Task<Quiz> createTask() {
return new Task<Quiz>() {
#Override
protected Quiz call() throws Exception {
// Severall calls to db here, Quiz object gets constructed
return quiz;
}
};
}
}

The problem in your code is, that you service methods are executed asynchronously.
You should return the Task<Quiz> instead of quiz and use that to update your frontend if the result is received (I have to few information to create an appropriate example for you).
Another option is to pass a callback to your service, which is invoked when the result is received instead of returning the quiz.
public void getQA(int id, QuizReceiver callback) throws SQLException {
try {
GetQuizService getQuizService = new GetQuizService();
getQuizService.restart();
getQuizService.setId(id);
getQuizService.setOnSucceeded(e -> {
callback.quizReceived(getQuizService.getValue());
});
} catch (Exception e) {
System.err.println(e);
}
return quiz;
}
public interface OuizReceiver {
void quizReceived(IQA quiz);
}

Java: message system needs to be able to pass various objects

I'm writing a messaging system to queue actions for my program to execute. I need to be able to pass various objects by the messages. I currently have a Msg object that accepts (Action enum, Data<?>...object). The Data object is intended to be a wrapper for any object I might pass.
Currently the Data object uses this code, with generics:
public class Data<T> {
private T data;
public Data(T data){
this.data = data;
}
public T getData(){
return data;
}
}
The Msg object takes Data<?>... type, so Msg has a Data<?>[] field.
If getData() is called on a Data<?> object, it returns the Object type. Obviously not ideal.
I need to be able to pass, say, Image objects as well as String objects. I'm certain there's a better way of passing arbitrary data.

The reason you're having trouble is that you're trying to get the static typing system of Java to do something that it can't. Once you convert from a Data<T> to a Data<?>, whatever T was is effectively lost. There's no clean way to get it back.
The quickest way to get it to work (from what you have right now) is to start throwing casts everywhere, like this:
Data<?> d = new Data("Hello");
String contents = (String)d.getData();
This is kind of a terrible idea, so let's go back to the drawing board.
If (ideally), you have all of the types you could ever need ahead of time (i.e. every Data is either a String or an Image or an Integer), then you can pretty easily (though it's a bit tedious) define a Sum type (aka a union if you're coming from C) of the different types of data you'll have to handle. As a class invariant, we assume that exactly one of the fields is non-null, and the rest are null. For this example I'll assume it can be either a String, an Image, or an Integer, but it's fairly simple to add or remove types from Data as necessary.
public class Data {
private Image imgData;
private String stringData;
private Integer intData;
public Data(Image img) {
this.imgData = img;
}
public Data(String stringData) {
this.stringData = stringData;
}
public Data(Integer intData) {
this.intData = intData;
}
public boolean isImage() {
return imageData != null;
}
public boolean isInteger() {
return intData != null;
}
public boolean isString() {
return stringData != null;
}
public Image asImage() {
if(! isImage()) throw new RuntimeException();
return imgData;
}
public Image asString() {
if(! isString()) throw new RuntimeException();
return stringData;
}
public Image asInt() {
if(! isInt()) throw new RuntimeException();
return intData;
}
}
One necessary side effect is that we cannot wrap null without causing exceptional behavior. Is this is desired, it isn't too difficult to modify the class to allow for it.
With this Data class, it's pretty easy to do if-else logic to parse it.
Data d = ....... //Get a data from somewhere
if(d.isImage()) {
Image img = d.asImage();
//...
} else if (d.isString()) {
String string = d.asString();
//...
} else if (d.isInteger()) {
Integer i = d.asInt();
//...
} else {
throw new RuntimeException("Illegal data " + d + " received");
}

If you call getData().getClass() you will get the class or type that was passed, which doesn't seem to me to be the same as an Object. You might not know what you are getting, but you can either find out or define a common interface for everything you might pass. You could for example, call toString() or getClass() on anything passed. Your question is that you are passing any conceivable object, so my question is what are you going to do with it? If you are going to serialize it into a database you don't need know anything about what type it is, otherwise you can test it or call a common interface.
public class PlayData {
class Msg {
private List<Data<?>> message = new ArrayList<Data<?>>();
public void addData(Data<?> datum) { message.add(datum); }
public void printTypes() { for ( Data<?> datum: message ) { System.out.println(datum.getData().getClass()); } }
}
class Data<T> {
private T value;
public Data(T value) { this.value = value; }
public T getData() { return value; }
}
class Listener {
public void receive(Msg msg) { msg.printTypes(); }
}
class Sender {
private Listener listener;
public Sender(Listener listener) { this.listener = listener; }
public void send(Msg msg) { listener.receive(msg); }
}
class MyPacket {
int i;
public MyPacket(int i) { this.i = i; }
}
public static void main(String[] args) throws Exception { new PlayData().run(); }
public void run() throws Exception {
Sender sender = new Sender(new Listener());
Msg msg = new Msg();
msg.addData(new Data<String>("testing") );
msg.addData(new Data<MyPacket>(new MyPacket(42)) );
sender.send(msg);
}
}

Creating a HashMap of type <String , Object>

In a previous post Creating a ToolTip Managed bean
I was able to create a manged bean to collect and display tooltip text with only a single lookup and store them in an Application Scope variable. This has worked great.
I am on the rather steep part of the JAVA learning curve so please forgive me.
I have another managed bean requirement to create a HashMap Application Scope but this time it needs to be of a type String, Object. The application is where I have a single 'master' database that contains most of the code, custom controls, and XPages. This Master Database will point to One or More application databases that will store the Notes Documents specific to the application in question. So I have created in the Master a series of Application Documents that specify the RepIDs of the Application, Help and Rules databases specific to the Application along with a number of other pieces of information about the Application. This should allow me to reuse the same custom control that will open the specific DB by passing it the Application Name. As an example the Master Design DB might point to "Purchasing", "Customer Complaints", "Travel Requests" etc.
The code below works to load and store the HashMap, but I am having trouble retrieving the the data.
The compiler shows two errors one at the public Object get(String key) method and the other at mapValue = this.internalMap.get(key); in the getAppRepID method I think that it is mainly syntax but not sure. I have commented the error in the code where it appears.
/**
*This Class makes the variables that define an application within Workflo!Approval
*available as an ApplicationScope variable.
*/
package ca.wfsystems.wfsAppUtils;
import lotus.domino.Base;
import lotus.domino.Session;
import lotus.domino.Database;
import lotus.domino.View;
import lotus.domino.NotesException;
import lotus.domino.ViewColumn;
import lotus.domino.ViewEntry;
import lotus.domino.ViewEntryCollection;
import lotus.domino.Name;
import java.io.Serializable;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Vector;
import com.ibm.xsp.extlib.util.ExtLibUtil;
/**
* #author Bill Fox Workflo Systems WFSystems.ca
* July 2014
* This class is provided as part of the Workflo!Approval Product
* and can be reused within this application.
* If copied to a different application please retain this attribution.
*
*/
public abstract class ApplicationUtils implements Serializable, Map<String, Object> {
private static final long serialVersionUID = 1L;
private Session s;
private Name serverName;
private String repID;
private String thisKey;
private ViewEntryCollection formVECol;
private Vector formNames;
private Database thisDB;
private Database appDB;
private View appView;
private View formView;
private ViewEntry formVE;
private ViewEntry tFormVE;
private ViewEntry ve;
private ViewEntry tVE;
private ViewEntryCollection veCol;
private final Map<String, Object> internalMap = new HashMap<String, Object>();
public ApplicationUtils() {
this.populateMap(internalMap);
}
private void populateMap(Map<String, Object> theMap) {
try{
s = ExtLibUtil.getCurrentSession();
//serverName = s.createName(s.getServerName());
thisDB = s.getCurrentDatabase();
appView = thisDB.getView("vwWFSApplications");
veCol = appView.getAllEntries();
ve = veCol.getFirstEntry();
ViewEntry tVE = null;
while (ve != null) {
rtnValue mapValue = new rtnValue();
tVE = veCol.getNextEntry(ve);
Vector colVal = ve.getColumnValues();
thisKey = colVal.get(0).toString();
mapValue.setRepID(colVal.get(2).toString());
// ...... load the rest of the values .......
theMap.put(thisKey, mapValue);
recycleObjects(ve);
ve = tVE;
}
}catch(NotesException e){
System.out.println(e.toString());
}finally{
recycleObjects(ve, veCol, appView, tVE);
}
}
public class rtnValue{
private String RepID;
private String HelpRepID;
private String RuleRepID;
private Vector FormNames;
public String getRepID() {
return RepID;
}
public void setRepID(String repID) {
RepID = repID;
}
public String getHelpRepID() {
return HelpRepID;
}
public void setHelpRepID(String helpRepID) {
HelpRepID = helpRepID;
}
public String getRuleRepID() {
return RuleRepID;
}
public void setRuleRepID(String ruleRepID) {
RuleRepID = ruleRepID;
}
public Vector getFormNames() {
return FormNames;
}
public void setFormNames(Vector formNames) {
FormNames = formNames;
}
}
public void clear() {
this.internalMap.clear();
this.populateMap(this.internalMap);
}
public boolean containsKey(Object key) {
return this.internalMap.containsKey(key);
}
public boolean containsValue(Object value) {
return this.internalMap.containsValue(value);
}
public Set<java.util.Map.Entry<String, Object>> entrySet() {
return this.internalMap.entrySet();
}
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) {
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
rtnValue newMap = new rtnValue();
return newMap;
}
}
public boolean isEmpty() {
return this.internalMap.isEmpty();
}
public Set<String> keySet() {
return this.internalMap.keySet();
}
public Object put(String key, Object value) {
return this.internalMap.put(key, value);
}
public Object remove(Object key) {
return this.internalMap.remove(key);
}
public int size() {
return this.internalMap.size();
}
public Collection<Object> values() {
return this.internalMap.values();
}
public void putAll(Map<? extends String, ? extends Object> m) {
this.internalMap.putAll(m);
}
public String getAppRepID(String key){
/*get the Replica Id of the application database
* not sure this is the correct way to call this
*/
rtnValue mapValue = new rtnValue();
mapValue = this.internalMap.get(key);
//error on line above Type Mismatch: can not convert Object to ApplicationUtils.rtnValue
String repID = mapValue.getRepID();
}
public static void recycleObjects(Object... args) {
for (Object o : args) {
if (o != null) {
if (o instanceof Base) {
try {
((Base) o).recycle();
} catch (Throwable t) {
// who cares?
}
}
}
}
}
}

For the get() method, the way I handle that kind of situation is create a variable of the correct data type as null, in my try/catch set the variable, and at the end return the variable. So:
Object retVal = null;
try....
return retVal;
For the other error, if you right-click on the error marker, it might give you the opportunity to cast the variable to rtnValue, so:
mapValue = (rtnValue) this.internalMap.get(key)
If you haven't got it, Head First Java was a useful book for getting my head around some Java concepts. It's also worth downloading the FindBugs plugin for Domino Designer from OpenNTF. It will identify errors as well as bad practices. Just ignore the errors in the "local" package!

The problem is that there is an execution path that do not return nothing
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) { // false
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
rtnValue newMap = new rtnValue();
return newMap;
}
}
if key is not present in the internalMap, nothing is thrown, then that method do not return anything.
To fix the problem, return the newMap at the end.
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) {
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
}
rtnValue newMap = new rtnValue();
return newMap;
}
You can inline the return to save the allocation (which is what the compiler will do anyway). I didn't do it just to make it clear in the example.
But still you have a compiler error in getAppRepID method. You are expecting a rtnValue but you send back an Object. You must cast there.
The appropriate way to handle this is, if you know that all values are of a given type, create the map with the proper type.

Have you tried making your internalMap a map of rtnValue instances (so )?

Spring SimpleJdbcCall default (optional) arguments

I am trying to invoke a stored procedure which has default (optional) arguments without passing them and it is not working. Essentially the same problem as described here.
My code:
SqlParameterSource in = new MapSqlParameterSource()
.addValue("ownname", "USER")
.addValue("tabname", cachedTableName)
.addValue("estimate_percent", 20)
.addValue("method_opt", "FOR ALL COLUMNS SIZE 1")
.addValue("degree", 0)
.addValue("granularity", "AUTO")
.addValue("cascade", Boolean.TRUE)
.addValue("no_invalidate", Boolean.FALSE)
.addValue("force", Boolean.FALSE);
And I get an exception:
Caused by: org.springframework.dao.InvalidDataAccessApiUsageException: Required input parameter 'PARTNAME' is missing
at org.springframework.jdbc.core.CallableStatementCreatorFactory$CallableStatementCreatorImpl.createCallableStatement(CallableStatementCreatorFactory.java:209)
Where PARTNAME is an optional parameter according to this. Also confirmed by the fact that I can run this procedure w/o the PARTNAME argument manually.

Ater giving up on this question and just passing all the parameters, including optional ones I ran into its inability to pass boolean arguments, because boolean is not an SQL data type, only PL/SQL.
So my current solution is that JDBC is not suited for running stored procedures and this is how I'm working around it:
jdbcTemplate.execute(
new CallableStatementCreator() {
public CallableStatement createCallableStatement(Connection con) throws SQLException{
CallableStatement cs = con.prepareCall("{call sys.dbms_stats.gather_table_stats(ownname=>user, tabname=>'" + cachedMetadataTableName + "', estimate_percent=>20, method_opt=>'FOR ALL COLUMNS SIZE 1', degree=>0, granularity=>'AUTO', cascade=>TRUE, no_invalidate=>FALSE, force=>FALSE) }");
return cs;
}
},
new CallableStatementCallback() {
public Object doInCallableStatement(CallableStatement cs) throws SQLException{
cs.execute();
return null; // Whatever is returned here is returned from the jdbcTemplate.execute method
}
}
);

Came up with a decent solution to this today, that copes with non-null defaults, and does not use fruity reflection techniques. It works by creating the metadata context for the function externally to retrieve all the parameter types and so forth, then constructing the SimpleJdbcCall manually from that.
First, create a CallMetaDataContext for the function:
CallMetaDataContext context = new CallMetaDataContext();
context.setFunction(true);
context.setSchemaName(schemaName);
context.setProcedureName(functionName);
context.initializeMetaData(jdbcTemplate.getDataSource());
context.processParameters(Collections.emptyList());
Next, create the SimpleJdbcCall, but force it to not do its own metadata lookup:
SimpleJdbcCall simpleJdbcCall = new SimpleJdbcCall(jdbcTemplate);
// This forces the call object to skip metadata lookup, which is the part that forces all parameters
simpleJdbcCall.setAccessCallParameterMetaData(false);
// Now go back to our previously created context and pull the parameters we need from it
simpleJdbcCall.addDeclaredParameter(context.getCallParameters().get(0));
for (int i = 0; i < params.length; ++i) {
simpleJdbcCall.addDeclaredParameter(context.getCallParameters().get(i));
}
// Call the function and retrieve the result
Map<String, Object> resultsMap = simpleJdbcCall
.withSchemaName(schemaName)
.withFunctionName(functionName)
.execute(params);
Object returnValue = resultsMap.get(context.getScalarOutParameterName());

I found solution for my case with SimpleJdbcCall and Spring 5.2.1, Java 8, Oracle 12.
You need to:
Use .withoutProcedureColumnMetaDataAccess()
Use .withNamedBinding()
Declare parameters, you know about in .declareParameters() call. Procedure will be called only with parameters, declared in this method. Default parameters, you dont want to set, arent writing here.
Example call is below
final String dataParamName = "P_DATA";
final String ageParamName = "P_AGE";
final String genderParamName = "P_GENDER";
final String acceptedParamName = "P_ACCEPTED";
SimpleJdbcCall simpleJdbcCall = new SimpleJdbcCall(getJdbcTemplate())
.withCatalogName("PKG_USER")
.withProcedureName("USER_CHECK")
.withoutProcedureColumnMetaDataAccess()
.withNamedBinding()
.declareParameters(
new SqlParameter(dataParamName, OracleTypes.VARCHAR),
new SqlParameter(ageParamName, OracleTypes.NUMBER),
new SqlParameter(genderParamName, OracleTypes.VARCHAR),
new SqlOutParameter(acceptedParamName, OracleTypes.NUMBER)
);
SqlParameterSource parameterSource = new MapSqlParameterSource()
.addValue(dataParamName, data)
.addValue(ageParamName, age)
.addValue(genderParamName, gender);
Map<String, Object> out = simpleJdbcCall.execute(parameterSource);

Here is a different approach that I have taken. I added the ability for the user to set the number of parameters they will be providing on the call. These will be the first n number of positional parameters. Any remaining parameters available in the stored-proc, will have to be set via the database's default value handling. This allows new parameters to be added to the end of the list with default values, or to be null-able, without breaking code that does not know to provide a value.
I sub-classed SimpleJdbcCall and added the methods to set the "maxParamCount". I also used a bit a evil reflection to set my sub-classed version of CallMetaDataContext.
public class MySimpleJdbcCall extends SimpleJdbcCall
{
private final MyCallMetaDataContext callMetaDataContext = new MyCallMetaDataContext();
public MySimpleJdbcCall(DataSource dataSource)
{
this(new JdbcTemplate(dataSource));
}
public MySimpleJdbcCall(JdbcTemplate jdbcTemplate)
{
super(jdbcTemplate);
try
{
// Access private field
Field callMetaDataContextField = AbstractJdbcCall.class.getDeclaredField("callMetaDataContext");
callMetaDataContextField.setAccessible(true);
// Make it non-final
Field modifiersField = Field.class.getDeclaredField("modifiers");
modifiersField.setAccessible(true);
modifiersField.setInt(callMetaDataContextField, callMetaDataContextField.getModifiers() & ~Modifier.FINAL);
// Set field
callMetaDataContextField.set(this, this.callMetaDataContext);
}
catch (NoSuchFieldException | IllegalAccessException ex)
{
throw new RuntimeException("Exception thrown overriding AbstractJdbcCall.callMetaDataContext field", ex);
}
}
public MySimpleJdbcCall withMaxParamCount(int maxInParamCount)
{
setMaxParamCount(maxInParamCount);
return this;
}
public int getMaxParamCount()
{
return this.callMetaDataContext.getMaxParamCount();
}
public void setMaxParamCount(int maxInParamCount)
{
this.callMetaDataContext.setMaxParamCount(maxInParamCount);
}
}
In my CallMetaDataContext sub-class, I store the maxInParamCount, and use it to trim the list of parameters known to exist in the stored-proc.
public class MyCallMetaDataContext extends CallMetaDataContext
{
private int maxParamCount = Integer.MAX_VALUE;
public int getMaxParamCount()
{
return maxParamCount;
}
public void setMaxParamCount(int maxInParamCount)
{
this.maxParamCount = maxInParamCount;
}
#Override
protected List<SqlParameter> reconcileParameters(List<SqlParameter> parameters)
{
List<SqlParameter> limittedParams = new ArrayList<>();
int paramCount = 0;
for(SqlParameter param : super.reconcileParameters(parameters))
{
if (!param.isResultsParameter())
{
paramCount++;
if (paramCount > this.maxParamCount)
continue;
}
limittedParams.add(param);
}
return limittedParams;
}
}
Use is basically the same except for seeting the max parameter count.
SimpleJdbcCall call = new MySimpleJdbcCall(jdbcTemplate)
.withMaxParamCount(3)
.withProcedureName("MayProc");
SMALL RANT: It's funny that Spring is well know for its IOC container. But, within its utility classes, I have to resort to reflection to provide an alternate implementation of a dependent class.

Was also struggling with the problem, and didn't want to deal with strings.
There could be more interesting solution, if we get default values from meta data, which spring doesn't care about in default implementation, but I simply put nulls there.
The solution came like the following:
Overridden simpleJdbcCall
private class JdbcCallWithDefaultArgs extends SimpleJdbcCall {
CallableStatementCreatorFactory callableStatementFactory;
public JdbcCallWithDefaultArgs(JdbcTemplate jdbcTemplate) {
super(jdbcTemplate);
}
#Override
protected CallableStatementCreatorFactory getCallableStatementFactory() {
return callableStatementFactory;
}
#Override
protected void onCompileInternal() {
callableStatementFactory =
new CallableStatementCreatorWithDefaultArgsFactory(getCallString(), this.getCallParameters());
callableStatementFactory.setNativeJdbcExtractor(getJdbcTemplate().getNativeJdbcExtractor());
}
#Override
public Map<String, Object> execute(SqlParameterSource parameterSource) {
((CallableStatementCreatorWithDefaultArgsFactory)callableStatementFactory).cleanupParameters(parameterSource);
return super.doExecute(parameterSource);
}
}
And overriden CallableStatementCreatorFactory
public class CallableStatementCreatorWithDefaultArgsFactory extends CallableStatementCreatorFactory {
private final Logger logger = LoggerFactory.getLogger(getClass());
private final List<SqlParameter> declaredParameters;
public CallableStatementCreatorWithDefaultArgsFactory(String callString, List<SqlParameter> declaredParameters) {
super(callString, declaredParameters);
this.declaredParameters = declaredParameters;
}
protected void cleanupParameters(SqlParameterSource sqlParameterSource) {
MapSqlParameterSource mapSqlParameterSource = (MapSqlParameterSource) sqlParameterSource;
Iterator<SqlParameter> declaredParameterIterator = declaredParameters.iterator();
Set<String> parameterNameSet = mapSqlParameterSource.getValues().keySet();
while (declaredParameterIterator.hasNext()) {
SqlParameter parameter = declaredParameterIterator.next();
if (!(parameter instanceof SqlOutParameter) &&
(!mapContainsParameterIgnoreCase(parameter.getName(), parameterNameSet))) {
logger.warn("Missing value parameter "+parameter.getName() + " will be replaced by null!");
mapSqlParameterSource.addValue(parameter.getName(), null);
}
}
}
private boolean mapContainsParameterIgnoreCase(String parameterName, Set<String> parameterNameSet) {
String lowerParameterName = parameterName.toLowerCase();
for (String parameter : parameterNameSet) {
if (parameter.toLowerCase().equals(lowerParameterName)) {
return true;
}
}
return false;
}
#Override
public void addParameter(SqlParameter param) {
this.declaredParameters.add(param);
}

I use this util method:
public <T> void setOptionalParameter(MapSqlParameterSource parameters, String name, T value) {
if (value == null)
parameters.addValue(name, value, Types.NULL);
else
parameters.addValue(name, value);
}

Cache in GWT app/widget with HTML5 localStorage

I am trying to incorporate a data cache for one of my GWT widgets.
I have a datasource interface/class which retrieves some data from my backend via RequestBuilder and JSON. Because I display the widget multiple times I only want to retrieve the data once.
So I tried to come with an app cache. The naive approach is to use a HashMap in a singleton object to store the data. However I also want to make use of HTML5's localStorage/sessionStorage if supported.
HTML5 localStorage only supports String values. So I have to convert my object into JSON and store as a string. However somehow I can't come up with a nice clean way of doing this. here is what I have so far.
I define a interface with two functions: fetchStatsList() fetches the list of stats that can be displayed in the widget and fetchStatsData() fetches the actual data.
public interface DataSource {
public void fetchStatsData(Stat stat,FetchStatsDataCallback callback);
public void fetchStatsList(FetchStatsListCallback callback);
}
The Stat class is a simple Javascript Overlay class (JavaScriptObject) with some getters (getName(), etc)
I have a normal non-cachable implementation RequestBuilderDataSource of my DataSource which looks like the following:
public class RequestBuilderDataSource implements DataSource {
#Override
public void fetchStatsList(final FetchStatsListCallback callback) {
// create RequestBuilderRequest, retrieve response and parse JSON
callback.onFetchStatsList(stats);
}
#Override
public void fetchStatsData(List<Stat> stats,final FetchStatsDataCallback callback) {
String url = getStatUrl(stats);
//create RequestBuilderRquest, retrieve response and parse JSON
callback.onFetchStats(dataTable); //dataTable is of type DataTable
}
}
I left out most of the code for the RequestBuilder as it is quite straightforward.
This works out of the box however the list of stats and also the data is retrieved everytime even tough the data is shared among each widget instance.
For supporting caching I add a Cache interface and two Cache implementations (one for HTML5 localStorage and one for HashMap):
public interface Cache {
void put(Object key, Object value);
Object get(Object key);
void remove(Object key);
void clear();
}
I add a new class RequestBuilderCacheDataSource which extends the RequestBuilderDataSource and takes a Cache instance in its constructor.
public class RequestBuilderCacheDataSource extends RequestBuilderDataSource {
private final Cache cache;
publlic RequestBuilderCacheDataSource(final Cache cache) {
this.cache = cache;
}
#Override
public void fetchStatsList(final FetchStatsListCallback callback) {
Object value = cache.get("list");
if (value != null) {
callback.fetchStatsList((List<Stat>)value);
}
else {
super.fetchStatsList(stats,new FetchStatsListCallback() {
#Override
public void onFetchStatsList(List<Stat>stats) {
cache.put("list",stats);
callback.onFetchStatsList(stats);
}
});
super.fetchStatsList(callback);
}
}
#Override
public void fetchStatsData(List<Stat> stats,final FetchStatsDataCallback callback) {
String url = getStatUrl(stats);
Object value = cache.get(url);
if (value != null) {
callback.onFetchStatsData((DataTable)value);
}
else {
super.fetchStatsData(stats,new FetchStatsDataCallback() {
#Override
public void onFetchStatsData(DataTable dataTable) {
cache.put(url,dataTable);
callback.onFetchStatsData(dataTable);
}
});
}
}
}
Basically the new class will lookup the value in the Cache and if it is not found it will call the fetch function in the parent class and intercept the callback to put it into the cache and then call the actual callback.
So in order to support both HTML5 localstorage and normal JS HashMap storage I created two implementations of my Cache interface:
JS HashMap storage:
public class DefaultcacheImpl implements Cache {
private HashMap<Object, Object> map;
public DefaultCacheImpl() {
this.map = new HashMap<Object, Object>();
}
#Override
public void put(Object key, Object value) {
if (key == null) {
throw new NullPointerException("key is null");
}
if (value == null) {
throw new NullPointerException("value is null");
}
map.put(key, value);
}
#Override
public Object get(Object key) {
// Check for null as Cache should not store null values / keys
if (key == null) {
throw new NullPointerException("key is null");
}
return map.get(key);
}
#Override
public void remove(Object key) {
map.remove(key);
}
#Override
public void clear() {
map.clear();
}
}
HTML5 localStorage:
public class LocalStorageImpl implements Cache{
public static enum TYPE {LOCAL,SESSION}
private TYPE type;
private Storage cacheStorage = null;
public LocalStorageImpl(TYPE type) throws Exception {
this.type = type;
if (type == TYPE.LOCAL) {
cacheStorage = Storage.getLocalStorageIfSupported();
}
else {
cacheStorage = Storage.getSessionStorageIfSupported();
}
if (cacheStorage == null) {
throw new Exception("LocalStorage not supported");
}
}
#Override
public void put(Object key, Object value) {
//Convert Object (could be any arbitrary object) into JSON
String jsonData = null;
if (value instanceof List) { // in case it is a list of Stat objects
JSONArray array = new JSONArray();
int index = 0;
for (Object val:(List)value) {
array.set(index,new JSONObject((JavaScriptObject)val));
index = index +1;
}
jsonData = array.toString();
}
else // in case it is a DataTable
{
jsonData = new JSONObject((JavaScriptObject) value).toString();
}
cacheStorage.setItem(key.toString(), jsonData);
}
#Override
public Object get(Object key) {
if (key == null) {
throw new NullPointerException("key is null");
}
String jsonDataString = cacheStorage.getItem(key.toString());
if (jsonDataString == null) {
return null;
}
Object data = null;
Object jsonData = JsonUtils.safeEval(jsonDataString);
if (!key.equals("list"))
data = DataTable.create((JavaScriptObject)data);
else if (jsonData instanceof JsArray){
JsArray<GenomeStat> jsonStats = (JsArray<GenomeStat>)jsonData;
List<GenomeStat> stats = new ArrayList<GenomeStat>();
for (int i = 0;i<jsonStats.length();i++) {
stats.add(jsonStats.get(i));
}
data = (Object)stats;
}
return data;
}
#Override
public void remove(Object key) {
cacheStorage.removeItem(key.toString());
}
#Override
public void clear() {
cacheStorage.clear();
}
public TYPE getType() {
return type;
}
}
The post got a little bit long but hopefully clarifies what I try to reach. It boils down to two questions:
Feedback on the design/architecture of this approach (for example subclassing RequestBilderDataSource for cache function, etc). Can this be improved (this is probably more related to general design than specifically GWT).
With the DefaultCacheImpl it is really easy to store and retrieve any arbitrary objects. How can I achieve the same thing with localStorage where I have to convert and parse JSON? I am using a DataTable which requires to call the DataTable.create(JavaScriptObject jso) function to work. How can I solve this without to many if/else and instance of checks?

My first thoughts: make it two layers of cache, not two different caches. Start with the in-memory map, so no serialization/deserialization is needed for reading a given object out, and so that changing an object in one place changes it in all. Then rely on the local storage to keep data around for the next page load, avoiding the need for pulling data down from the server.
I'd tend to say skip session storage, since that doesn't last long, but it does have its benefits.
For storing/reading data, I'd encourage checking out AutoBeans instead of using JSOs. This way you could support any type of data (that can be stored as an autobean) and could pass in a Class param into the fetcher to specify what kind of data you will read from the server/cache, and decode the json to a bean in the same way. As an added bonus, autobeans are easier to define - no JSNI required. A method could look something like this (note that In DataSource and its impl, the signature is different).
public <T> void fetch(Class<T> type, List<Stat> stats, Callback<T, Throwable> callback);
That said, what is DataTable.create? If it is already a JSO, you can just cast to DataTable as you (probably) normally do when reading from the RequestBuilder data.
I would also encourage not returning a JSON array directly from the server, but wrapping it in an object, as a best practice to protect your users' data from being read by other sites. (Okay, on re-reading the issues, objects aren't great either). Rather than discussing it here, check out JSON security best practices?
So, all of that said, first define the data (not really sure how this data is intended to work, so just making up as I go)
public interface DataTable {
String getTableName();
void setTableName(String tableName);
}
public interface Stat {// not really clear on what this is supposed to offer
String getKey();
void setKey(String key);
String getValue();
String setValue(String value);
}
public interface TableCollection {
List<DataTable> getTables();
void setTables(List<DataTable> tables);
int getRemaining();//useful for not sending all if you have too much?
}
For autobeans, we define a factory that can create any of our data when given a Class instance and some data. Each of these methods can be used as a sort of constructor to create a new instance on the client, and the factory can be passed to AutoBeanCodex to decode data.
interface DataABF extends AutoBeanFactory {
AutoBean<DataTable> dataTable();
AutoBean<Stat> stat();
AutoBean<TableCollection> tableCollection();
}
Delegate all work of String<=>Object to AutoBeanCodex, but you probably want some simple wrapper around it to make it easy to call from both the html5 cache and from the RequestBuilder results. Quick example here:
public class AutoBeanSerializer {
private final AutoBeanFactory factory;
public AutoBeanSerializer(AutoBeanFactory factory) {
this.factory = factory;
}
public String <T> encodeData(T data) {
//first, get the autobean mapped to the data
//probably throw something if we can't find it
AutoBean<T> autoBean = AutoBeanUtils.getAutoBean(data);
//then, encode it
//no factory or type needed here since the AutoBean has those details
return AutoBeanCodex.encode(autoBean);
}
public <T> T decodeData(Class<T> dataType, String json) {
AutoBean<T> bean = AutoBeanCodex.decode(factory, dataType, json);
//unwrap the bean, and return the actual data
return bean.as();
}
}