HTTP-RPC / Http Rpc
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Introduction
HTTP-RPC is an open-source framework for creating and consuming RESTful and REST-like web services in Java. It is extremely lightweight and requires only a Java runtime environment and a servlet container. The entire framework is about 100KB in size, making it an ideal choice for applications where a minimal footprint is desired.
This guide introduces the HTTP-RPC framework and provides an overview of its key features.
Contents
Getting HTTP-RPC
HTTP-RPC is distributed via Maven Central:
- org.httprpc:httprpc-client - provides support for consuming web services and interacting with common file formats and relational databases (Java 8 or later required)
- org.httprpc:httprpc-server - depends on client; provides support for implementing web services (Java Servlet specification 3.1 or later required)
NOTE The legacy org.httprpc:httprpc
artifact is deprecated. org.httprpc:httprpc-client
or org.httprpc:httprpc-server
should be used for new development.
HTTP-RPC Classes
Classes provided by the HTTP-RPC framework include:
- WebService - abstract base class for web services
- WebServiceProxy - client-side invocation proxy for web services
- JSONEncoder and JSONDecoder - encodes/decodes an object hierarchy to/from JSON
- CSVEncoder and CSVDecoder - encodes/decodes an iterable sequence of values to/from CSV
- TextEncoder and TextDecoder - encodes/decodes text content
- TemplateEncoder - encodes an object hierarchy using a template document
- BeanAdapter - map adapter for Java beans
- ResultSetAdapter and Parameters - iterable adapter for JDBC result sets/applies named parameter values to prepared statements
- QueryBuilder - programmatically constructs a SQL query
- ElementAdapter - map adapter for XML elements
- ResourceBundleAdapter - map adapter for resource bundles
- StreamAdapter - iterable adapter for streams
- Collections - utility methods for working with collections
Each is discussed in more detail in the following sections.
WebService
WebService
is an abstract base class for web services. It extends the similarly abstract HttpServlet
class provided by the servlet API.
Service operations are defined by adding public methods to a concrete service implementation. Methods are invoked by submitting an HTTP request for a path associated with a servlet instance. Arguments are provided either via the query string or in the request body, like an HTML form. WebService
converts the request parameters to the expected argument types, invokes the method, and writes the return value to the output stream as JSON. Service classes must be compiled with the -parameters
flag so the names of their method parameters are available at runtime.
The RequestMethod
annotation is used to associate a service method with an HTTP verb such as GET
or POST
. The optional ResourcePath
annotation can be used to associate the method with a specific path relative to the servlet. If unspecified, the method is associated with the servlet itself. If no matching handler method is found for a given request, the default handler (e.g. doGet()
) is called.
Multiple methods may be associated with the same verb and path. WebService
selects the best method to execute based on the provided argument values. For example, the following service class implements some simple addition operations:
@WebServlet(urlPatterns={"/math/*"}, loadOnStartup = 1)
public class MathService extends WebService {
@RequestMethod("GET")
@ResourcePath("sum")
public double getSum(double a, double b) {
return a + b;
}
@RequestMethod("GET")
@ResourcePath("sum")
public double getSum(List<Double> values) {
double total = 0;
for (double value : values) {
total += value;
}
return total;
}
}
The following HTTP request would cause the first method to be invoked:
GET /math/sum?a=2&b=4
This request would invoke the second method:
GET /math/sum?values=1&values=2&values=3
In either case, the service would return the value 6 in response.
Method Arguments
Method arguments may be any of the following types:
String
-
Byte
/byte
-
Short
/short
-
Integer
/int
-
Long
/long
-
Float
/float
-
Double
/double
-
Boolean
/boolean
-
java.util.Date
(from a long value representing epoch time in milliseconds) -
java.time.Instant
("yyyy-mm-ddThh:mm:ss[.sss]Z") -
java.time.LocalDate
("yyyy-mm-dd") -
java.time.LocalTime
("hh:mm") -
java.time.LocalDateTime
("yyyy-mm-ddThh:mm") java.net.URL
java.util.List
Missing or null
values are automatically converted to 0
or false
for primitive types.
List
arguments represent multi-value parameters. List values are automatically converted to their declared types (e.g. List<Double>
).
URL
and List<URL>
arguments represent file uploads. They may be used only with POST
requests submitted using the multi-part form data encoding. For example:
@WebServlet(urlPatterns={"/upload/*"}, loadOnStartup = 1)
@MultipartConfig
public class FileUploadService extends WebService {
@RequestMethod("POST")
public void upload(URL file) throws IOException {
try (InputStream inputStream = file.openStream()) {
...
}
}
@RequestMethod("POST")
public void upload(List<URL> files) throws IOException {
for (URL file : files) {
try (InputStream inputStream = file.openStream()) {
...
}
}
}
}
The methods could be invoked using this HTML form:
<form action="/upload" method="post" enctype="multipart/form-data">
<input type="file" name="file"/><br/>
<input type="file" name="files" multiple/><br/>
<input type="submit"/><br/>
</form>
If no method is found that matches the provided arguments, an HTTP 405 response is returned.
Path Variables
Path variables may be specified by a "?" character in the resource path. For example:
@RequestMethod("GET")
@ResourcePath("contacts/?/addresses/?")
public List<Address> getContactAddresses() { ... }
The getKey()
method returns the value of a path variable associated with the current request:
protected String getKey(int index) { ... }
For example, given the following request:
GET /contacts/jsmith/addresses/home
the value of the key at index 0 would be "jsmith", and the value at index 1 would be "home".
Named Variables
Path variables can optionally be assigned a name by appending a colon and key name to the "?" character:
@RequestMethod("GET")
@ResourcePath("contacts/?:contactID/addresses/?:addressType")
public List<Address> getContactAddresses() { ... }
A named variable can be retrieved via this getKey()
overload:
protected String getKey(String name) { ... }
For example, given the preceding request, the key with name "contactID" would be "jsmith" and the key with name "addressType" would be "home".
Custom Body Content
The Content
annotation can be used to associate custom body content with a service method. It defines a single value()
attribute representing the expected body type. Annotated methods can access the decoded content via the getBody()
method.
For example, the following service method might be used to create a new account record using data passed in the request body:
@RequestMethod("POST")
@Content(Account.class)
public createAccount() {
Account account = getBody();
...
}
By default, body data is assumed to be JSON. However, subclasses can override the decodeBody()
method to support other representations. If the provided data cannot be deserialized to the specified type, an HTTP 415 response will be returned.
Return Values
Return values are converted to their JSON equivalents as follows:
-
CharSequence
: string -
Number
: number -
Boolean
: true/false -
Enum
: string -
java.util.Date
: number representing epoch time in milliseconds -
java.time.TemporalAccessor
: string -
java.net.URL
: string -
Iterable
: array -
java.util.Map
or Java bean: object
By default, an HTTP 200 response is returned when a service method completes successfully. However, if a method returns void
or Void
, an HTTP 204 response will be returned. If a method returns null
, HTTP 404 will be returned.
Custom Result Encodings
Although return values are encoded as JSON by default, subclasses can override the encodeResult()
method of the WebService
class to support alternative encodings. See the method documentation for more information.
Request and Repsonse Properties
WebService
provides the following methods to allow a service method to access the request and response objects associated with the current invocation:
protected HttpServletRequest getRequest() { ... }
protected HttpServletResponse getResponse() { ... }
For example, a service might use the request to get the name of the current user, or use the response to return a custom header or status code.
The response object can also be used to produce a custom result. If a service method commits the response by writing to the output stream, the method's return value (if any) will be ignored by WebService
. This allows a service to return content that cannot be easily represented as JSON, such as image data.
Authorization
Service requests can be authorized by overriding the following method:
protected boolean isAuthorized(HttpServletRequest request, Method method) { ... }
The first argument contains the current request, and the second the service method to be invoked. If isAuthorized()
returns true
(the default), method execution will proceed. Otherwise, the method will not be invoked, and an HTTP 403 response will be returned.
Exceptions
If an exception is thrown by a service method and the response has not yet been committed, the exception message (if any) will be returned as plain text in the response body. Error status will be returned as shown below:
-
IllegalArgumentException
orUnsupportedOperationException
- HTTP 403 (forbidden) -
NoSuchElementException
- HTTP 404 (not found) -
IllegalStateException
- HTTP 409 (conflict) - Any other exception type - HTTP 500 (internal server error)
API Documentation
API documentation can be viewed by appending "?api" to a service URL; for example:
GET /math?api
Methods are grouped by resource path. Parameter, body, and return types are encoded as follows:
-
Object
: "any" -
Void
orvoid
: "void" -
Byte
orbyte
: "byte" -
Short
orshort
: "short" -
Integer
orint
: "integer" -
Long
orlong
: "long" -
Float
orfloat
: "float" -
Double
ordouble
: "double" - Any other
Number
: "number" -
CharSequence
: "string" -
java.util.Date
: "date" -
java.time.Instant
: "instant" -
java.time.LocalDate
: "date-local" -
java.time.LocalTime
: "time-local" -
java.time.LocalDateTime
: "datetime-local" -
java.net.URL
: "url" -
java.lang.Iterable
: "[element type]" -
java.util.Map
: "[key type: value type]"
Any other type is described by its simple class name.
Implementations can provide additional information about service types and operations using the Description
annotation. For example:
@WebServlet(urlPatterns={"/math/*"}, loadOnStartup = 1)
@Description("Math example service.")
public class MathService extends WebService {
@RequestMethod("GET")
@ResourcePath("sum")
@Description("Calculates the sum of two numbers.")
public double getSum(
@Description("The first number.") double a,
@Description("The second number.") double b
) {
return a + b;
}
...
}
The Description
annotation can also be applied to bean types and properties:
@Description("Represents an item in a product catalog.")
public class Item {
...
@Description("The item's description.")
public String getDescription() {
return description;
}
@Description("The item's price.")
public double getPrice() {
return price;
}
}
as well as enumerated types:
@Description("Represents a size option.")
public enum Size {
@Description("A small size.")
SMALL,
@Description("A medium size.")
MEDIUM,
@Description("A large size.")
LARGE
}
If a method is tagged with the Deprecated
annotation, it will be identified as such in the output.
WebServiceProxy
The WebServiceProxy
class is used to issue API requests to a server. It provides a single constructor that accepts the following arguments:
-
method
- the HTTP method to execute -
url
- the URL of the requested resource
Request headers and arguments are specified via the setHeaders()
and setArguments()
methods, respectively. Custom body content can be provided via the setBody()
method. When specified, body content is serialized as JSON; however, the setRequestHandler()
method can be used to facilitate custom request encodings.
Like HTML forms, arguments are submitted either via the query string or in the request body. Arguments for GET
, PUT
, and DELETE
requests are always sent in the query string. POST
arguments are typically sent in the request body, and may be submitted as either "application/x-www-form-urlencoded" or "multipart/form-data" (specified via the proxy's setEncoding()
method). However, if a custom body is provided either via setBody()
or by a custom request handler, POST
arguments will be sent in the query string.
Any value may be used as an argument. However, Date
instances are automatically converted to a long value representing epoch time. Additionally, Iterable
instances represent multi-value parameters and behave similarly to <select multiple>
tags in HTML. When using the multi-part encoding, instances of URL
represent file uploads and behave similarly to <input type="file">
tags in HTML forms.
Service operations are invoked via one of the following methods:
public <T> T invoke() throws IOException { ... }
public <T> T invoke(Class<? extends T> type) throws IOException { ... }
public <T> T invoke(ResponseHandler<T> responseHandler) throws IOException { ... }
The first two versions automatically deserialize a successful JSON response (if any). The third allows a caller to provide a custom response handler:
public interface ResponseHandler<T> {
T decodeResponse(InputStream inputStream, String contentType) throws IOException;
}
If a service returns an error response, the default error handler will throw a WebServiceException
. If the content type of the error response is "text/*", the deserialized response body will be provided in the exception message. A custom error handler can be supplied via the setErrorHandler()
method.
The following code snippet demonstrates how WebServiceProxy
might be used to access the operations of the simple math service discussed earlier:
WebServiceProxy webServiceProxy = new WebServiceProxy("GET", new URL(baseURL, "math/sum"));
// GET /math/sum?a=2&b=4
webServiceProxy.setArguments(mapOf(
entry("a", 4),
entry("b", 2)
));
System.out.println(webServiceProxy.invoke(Double.class)); // 6.0
// GET /math/sum?values=1&values=2&values=3
webServiceProxy.setArguments(mapOf(
entry("values", listOf(1, 2, 3))
));
System.out.println(webServiceProxy.invoke(Double.class)); // 6.0
Fluent Invocation
WebServiceProxy
also supports a fluent (i.e. chained) invocation model. For example, the following code is equivalent to the previous example:
// GET /math/sum?a=2&b=4
System.out.println(WebServiceProxy.get(baseURL, "math/sum").setArguments(mapOf(
entry("a", 4),
entry("b", 2)
)).invoke(Double.class)); // 6.0
// GET /math/sum?values=1&values=2&values=3
System.out.println(WebServiceProxy.get(baseURL, "math/sum").setArguments(mapOf(
entry("values", listOf(1, 2, 3))
)).invoke(Double.class)); // 6.0
POST, PUT, and DELETE operations are also supported.
JSONEncoder and JSONDecoder
The JSONEncoder
class is used internally by WebService
and WebServiceProxy
to serialize request and response data. However, it can also be used by application code. For example:
Map<String, Object> map = mapOf(
entry("vegetables", listOf(
"carrots",
"peas",
"potatoes"
)),
entry("desserts", listOf(
"cookies",
"cake",
"ice cream"
))
);
JSONEncoder jsonEncoder = new JSONEncoder();
jsonEncoder.write(map, System.out);
This code would produce the following output:
{
"vegetables": [
"carrots",
"peas",
"potatoes"
],
"desserts": [
"cookies",
"cake",
"ice cream"
]
}
Values are converted to their JSON equivalents as described earlier. Unsupported types are treated as null
.
JSONDecoder
deserializes a JSON document into a Java object hierarchy. JSON values are mapped to their Java equivalents as follows:
- string:
String
- number:
Number
- true/false:
Boolean
- array:
java.util.List
- object:
java.util.Map
For example, given the following document:
[
{
"name": "January",
"days": 31
},
{
"name": "February",
"days": 28
},
{
"name": "March",
"days": 31
},
...
]
JSONDecoder
could be used to parse the data into a list of maps as shown below:
JSONDecoder jsonDecoder = new JSONDecoder();
List<Map<String, Object>> months = jsonDecoder.read(inputStream);
for (Map<String, Object> month : months) {
System.out.println(String.format("%s has %d days", month.get("name"), month.get("days")));
}
CSVEncoder and CSVDecoder
The CSVEncoder
class can be used to serialize a sequence of map values to CSV. For example, the following code could be used to export the month/day list from the previous example as CSV. The string values passed to the constructor represent the columns in the output document and the map keys to which those columns correspond:
CSVEncoder csvEncoder = new CSVEncoder(listOf("name", "days"));
csvEncoder.write(months, System.out);
This code would produce the following output:
"name","days"
"January",31
"February",28
"March",31
...
String values are automatically wrapped in double-quotes and escaped. Instances of java.util.Date
are encoded as a long value representing epoch time. All other values are encoded via toString()
.
CSVDecoder
deserializes a CSV document into an iterable sequence of maps. Rather than loading the entire payload into memory and returning the data as a list, CSVDecoder
returns the data as a forward-scrolling cursor, allowing consumers to process rows as soon as they are read.
For example, given the CSV above as input, the following code would produce the same results as JSONDecoder
example:
CSVDecoder csvDecoder = new CSVDecoder();
Iterable<Map<String, String>> months = csvDecoder.read(inputStream);
for (Map<String, String> month : months) {
System.out.println(String.format("%s has %d days", month.get("name"), month.get("days")));
}
Columns with empty headings are ignored. Empty field values are treated as null.
TextEncoder and TextDecoder
The TextEncoder
and TextDecoder
classes can be used to serialize and deserialize plain text content, respectively. For example:
TextEncoder textEncoder = new TextEncoder();
try (FileOutputStream outputStream = new FileOutputStream(file)) {
textEncoder.write("Hello, World!", outputStream);
}
TextDecoder textDecoder = new TextDecoder();
String text;
try (FileInputStream inputStream = new FileInputStream(file)) {
text = textDecoder.read(inputStream); // Hello, World!
}
TemplateEncoder
The TemplateEncoder
class transforms an object hierarchy into an output format using a template document. Template syntax is based loosely on the Mustache format and supports most Mustache features.
TemplateEncoder
provides the following constructors:
public TemplateEncoder(URL url) { ... }
public TemplateEncoder(URL url, Charset charset) { ... }
The first argument specifies the URL of the template document (typically as a resource on the application's classpath). The escape modifier corresponding to the document's extension (if any) will be applied by default. The optional second argument represents the character encoding used by the template document. If unspecified, UTF-8 is assumed.
Templates are applied using one of the following methods:
public void write(Object value, OutputStream outputStream) { ... }
public void write(Object value, OutputStream outputStream, Locale locale) { ... }
public void write(Object value, OutputStream outputStream, Locale locale, TimeZone timeZone) { ... }
public void write(Object value, Writer writer) { ... }
public void write(Object value, Writer writer, Locale locale) { ... }
public void write(Object value, Writer writer, Locale locale, TimeZone timeZone) { ... }
The first argument represents the value to write (i.e. the data dictionary), and the second the output destination. The optional third and fourth arguments represent the target locale and time zone, respectively. If unspecified, system defaults are used.
For example, the following code snippet applies a template named example.txt to a map instance:
Map<String, Object> map = mapOf(
entry("a", "hello"),
entry("b", 123),
entry("c", true)
);
TemplateEncoder templateEncoder = new TemplateEncoder(getClass().getResource("example.txt"));
templateEncoder.write(map, System.out);
Custom Modifiers
Modifiers are created by implementing the TemplateEncoder.Modifier
interface, which defines the following method:
Object apply(Object value, String argument, Locale locale, TimeZone timeZone);
The first argument to this method represents the value to be modified, and the second is the optional argument value following the "=" character in the modifier string. If an argument is not specified, this value will be null
. The third argument contains the encoder's locale.
Custom modifiers are added to a template encoder instance via the getModifiers()
method. For example, the following code creates a modifier that converts values to uppercase:
templateEncoder.getModifiers().put("uppercase", (value, argument, locale, timeZone) -> value.toString().toUpperCase(locale));
Note that modifiers must be thread-safe, since they are shared and may be invoked concurrently by multiple encoder instances.
BeanAdapter
The BeanAdapter
class provides access to the properties of a Java bean instance via the Map
interface. For example, the following class might be used to represent a node in a hierarchical object graph:
public class TreeNode {
private String name;
private List<TreeNode> children;
public TreeNode(String name, List<TreeNode> children) {
this.name = name;
this.children = children;
}
public String getName() {
return name;
}
public List<TreeNode> getChildren() {
return children;
}
}
A simple tree structure could be created and serialized to JSON like this:
TreeNode root = TreeNode("Seasons", listOf(
new TreeNode("Winter", listOf(
new TreeNode("January", null),
new TreeNode("February", null),
new TreeNode("March", null)
)),
new TreeNode("Spring", listOf(
new TreeNode("April", null),
new TreeNode("May", null),
new TreeNode("June", null)
)),
new TreeNode("Summer", listOf(
new TreeNode("July", null),
new TreeNode("August", null),
new TreeNode("September", null)
)),
new TreeNode("Fall", listOf(
new TreeNode("October", null),
new TreeNode("November", null),
new TreeNode("December", null)
))
));
JSONEncoder jsonEncoder = new JSONEncoder();
jsonEncoder.write(new BeanAdapter(root), System.out);
or used as a data dictionary for a template document like this:
TemplateEncoder templateEncoder = new TemplateEncoder(getClass().getResource("tree.html"));
templateEncoder.write(new BeanAdapter(root), System.out);
Typed Access
BeanAdapter
can also be used to facilitate type-safe access to loosely typed data structures, such as decoded JSON objects:
public static <T> T adapt(Object value, Type type) { ... }
For example, given this interface:
public interface TreeNode {
String getName();
List<TreeNode> getChildren();
}
the following code could be used to translate the JSON data generated by the previous example into a collection of TreeNode
instances:
JSONDecoder jsonDecoder = new JSONDecoder();
Map<String, Object> map = jsonDecoder.read(inputStream);
TreeNode root = BeanAdapter.adapt(map, TreeNode.class);
System.out.println(root.getName()); // "Seasons"
System.out.println(root.getChildren().get(0).getName()); // "Winter"
System.out.println(root.getChildren().get(0).getChildren().get(0).getName()); // "January"
See the class documentation for more information.
Custom Property Keys
The Key
annotation can be used to associate a custom name with a bean property. The provided value will be used in place of the property name when reading or writing property values. For example:
public class Person {
private String firstName = null;
@Key("first_name")
public String getFirstName() {
return firstName;
}
@Key("first_name")
public void setFirstName(String firstName) {
this.firstName = firstName;
}
}
ResultSetAdapter and Parameters
The ResultSetAdapter
class provides access to the contents of a JDBC result set via the Iterable
interface. Access to individual rows is provided via the Map
interface:
public class ResultSetAdapter implements Iterable<Map<String, Object>>, AutoCloseable { ... }
ResultSetAdapter
also implements AutoCloseable
and ensures that the underlying result set is closed when the adapter is closed.
For example, the following code could be used to serialize the results of a database query to JSON:
try (ResultSetAdapter resultSetAdapter = new ResultSetAdapter(statement.executeQuery())) {
JSONEncoder jsonEncoder = new JSONEncoder();
jsonEncoder.write(resultSetAdapter, System.out);
}
The Parameters
class is used to simplify execution of prepared statements. It provides a means for executing statements using named parameter values rather than indexed arguments. Parameter names are specified by a leading ":" character. For example:
select name from pet where owner = :owner
Colons within single quotes and occurrences of two successive unquoted colons ("::") are ignored.
The parse()
method is used to create a Parameters
instance from a SQL statement. It takes a string or reader containing the SQL text as an argument; for example:
Parameters parameters = Parameters.parse(sql);
The getSQL()
method returns the parsed SQL in standard JDBC syntax:
select name from pet where owner = ?
This value is used to create the actual prepared statement. Arguments values are specified via the apply()
method:
PreparedStatement statement = connection.prepareStatement(parameters.getSQL());
parameters.apply(statement, mapOf(
entry("owner", "Gwen")
));
Once applied, the statement can be executed:
ResultSetAdapter resultSetAdapter = new ResultSetAdapter(statement.executeQuery());
QueryBuilder
The QueryBuilder
class provides a fluent API for programmatically constructing SQL queries.
For example, the query from the previous section could be created as follows using QueryBuilder
:
String sql = QueryBuilder.select("name", "species", "sex", "birth")
.from("pet")
.where("owner = :owner").toString();
Insert, update, and delete operations are also supported. In general, string values provided to the insertInto()
and set()
methods are wrapped in single quotes, and any embdedded single quotes are replaced with two successive single quotes. However, any string that starts with ":" or is equal to "?" is assumed to be a parameter reference and is not escaped.
If an instance of QueryBuilder
is passed to insertInto()
or set()
, it is considered a subquery and wrapped in parentheses.
ElementAdapter
The ElementAdapter
class provides access to the contents of an XML DOM Element
via the Map
interface. The resulting map can then be transformed to another representation via a template document or accessed via a strongly typed interface proxy, as described earlier.
For example, the following markup might be used to represent the status of a bank account:
<account id="101">
<holder>
<firstName>John</firstName>
<lastName>Smith</lastName>
</holder>
<transactions>
<credit>
<amount>100.00</amount>
<date>10/5/2020</date>
</credit>
<credit>
<amount>50.00</amount>
<date>10/12/2020</date>
</credit>
<debit>
<amount>25.00</amount>
<date>10/14/2020</date>
</debit>
<credit>
<amount>75.00</amount>
<date>10/19/2020</date>
</credit>
</transactions>
</account>
This code could be used to display the account holder's name:
ElementAdapter accountAdapter = new ElementAdapter(document.getDocumentElement());
Map<String, Object> holder = (Map<String, Object>)accountAdapter.get("holder");
System.out.println(String.format("%s, %s", holder.get("lastName"), holder.get("firstName")));
Attribute values can be obtained by prepending an "@" symbol to the attribute name:
System.out.println(accountAdapter.get("@id")); // "101"
A list of sub-elements can be obtained by appending an asterisk to the element name:
Map<String, Object> transactions = (Map<String, Object>)accountAdapter.get("transactions");
List<Map<String, Object>> credits = (List<Map<String, Object>>)transactions.get("credit*");
for (Map<String, Object> credit : credits) {
...
}
Finally, the text content of an element can be obtained by calling toString()
on the adapter instance:
System.out.println(credit.get("amount").toString());
System.out.println(credit.get("date").toString());
ResourceBundleAdapter
The ResourceBundleAdapter
class provides access to the contents of a resource bundle via the Map
interface. It can be used to localize the contents of a template document, for example:
<table>
<!-- {{?headings}} -->
<tr>
<td>{{name}}</td>
<td>{{description}}</td>
<td>{{quantity}}</td>
</tr>
<!-- {{/headings}} -->
<!-- {{#items}} -->
<tr>
<td>{{name}}</td>
<td>{{description}}</td>
<td>{{quantity}}</td>
</tr>
<!-- {{/items}} -->
</table>
TemplateEncoder templateEncoder = new TemplateEncoder(getClass().getResource("list.html"));
ResourceBundle resourceBundle = ResourceBundle.getBundle(getClass().getPackage().getName() + ".headings");
templateEncoder.write(mapOf(
entry("headings", new ResourceBundleAdapter(resourceBundle)),
entry("items", items)
), System.out);
StreamAdapter
The StreamAdapter
class provides access to the contents of a stream via the Iterable
interface. For example, it can be used to serialize the result of a stream operation without needing to first collect the results, which could be expensive if the stream is large:
List<Integer> values = listOf(1, 2, 3);
JSONEncoder jsonEncoder = new JSONEncoder(true);
jsonEncoder.write(new StreamAdapter<>(values.stream().map(element -> element * 2)), System.out);
StreamAdapter
also implements AutoCloseable
and ensures that the underlying stream is closed when the adapter is closed.
Collections
The Collections
class provides a set of static utility methods for instantiating immutable list and map values:
public static <E> List<E> listOf(E... elements) { ... }
public static <K, V> Map<K, V> mapOf(Map.Entry<K, V>... entries) { ... }
public static <K, V> Map.Entry<K, V> entry(K key, V value) { ... }
These methods are provided primarily as a convenience for applications using Java 8. Applications targeting Java 9 and higher can use the standard List.of()
and Map.of()
methods provided by the JDK.
Collections
additionally provides the valueAt()
method, which can be used to access nested values in an object hierarchy. For example:
Map<String, Object> map = mapOf(
entry("a", mapOf(
entry("b", mapOf(
entry("c", listOf(
1, 2, 3
))
))
))
);
int value = valueAt(map, "a", "b", "c", 1); // 2
Kotlin Support
In addition to Java, HTTP-RPC web services can be implemented using the Kotlin programming language. For example, the following service provides some basic information about the host system:
@WebServlet(urlPatterns = ["/system-info/*"], loadOnStartup = 1)
@Description("System info service.")
class SystemInfoService : WebService() {
class SystemInfo(
val availableProcessors: Int,
val freeMemory: Long,
val totalMemory: Long
)
@RequestMethod("GET")
@Description("Returns system info.")
fun getSystemInfo(): SystemInfo {
val runtime = Runtime.getRuntime()
return SystemInfo(
runtime.availableProcessors(),
runtime.freeMemory(),
runtime.totalMemory()
)
}
}
A response produced by the service might look like this:
{
"availableProcessors": 16,
"freeMemory": 85845656,
"totalMemory": 134217728
}
Additional Information
This guide introduced the HTTP-RPC framework and provided an overview of its key features. For additional information, see the examples.