org.graalvm.polyglot

Class Context

java.lang.Object

org.graalvm.polyglot.Context

All Implemented Interfaces:

AutoCloseable

public final class Context
extends Object
implements AutoCloseable

A polyglot context for Graal guest languages that allows to evaluate code. A polyglot context represents the global runtime state of all installed and permitted languages. Permitted languages are initialized lazily, when they are used for the first time. For many context operations, a language identifier needs to be specified. A language identifier is unique for each language.

Evaluation

A context allows to evaluate a guest language source code using Context.eval(Source). This is possible by evaluating Source objects or a given language identifier and code String. The evaluation returns either the result value or throws a PolyglotException if a guest language error occurs.

Example for evaluation of a fragment of JavaScript code with a new context:

 Context context = Context.create();
 Value result = context.eval("js", "42");
 assert result.asInt() == 42;
 context.close();
 

In this example:

• First we create a new context with all permitted languages.
• Next, we evaluate the expression "42" with language "js", which is the language identifier for JavaScript. Since this is the first time we access JavaScript, it automatically gets initialized.
• Then, we assert the result value by converting the result value as primitive int .
• Finally, if the context is no longer needed, it is necessary to close it to ensure that all resources are freed. Contexts are also AutoCloseable for use with the Java try-with-resources statement.

Configuration

Contexts may be created either with default configuration using the create method or with custom configuration using the builder. Both methods allow to specify a subset of the installed languages as permitted languages. If no language is specified then all installed languages are permitted. Using the builder method input, error and output streams, options, and application arguments may be configured.

Options may be specified for languages, instruments, the engine and the compiler. For language options, the option key consists of the language id plus a dot followed by the option name (e.g. "js.Strict"). For most languages the option names start with an upper-case letter by convention. A list of available options may be received using Language.getOptions(). Instrument options are structured in the same way as language options but start with the instrument id instead.

If system properties are enabled, which they are by default, then all polyglot options maybe specified with the prefix "polyglot." (e.g. "-Dpolyglot.js.Strict=true"). The system properties are read only once when the context or engine instance is created. After that, changes to the system properties have no affect.

Each Graal language performs an initialization step before it can be used to execute code, after which it remains initialized for the lifetime of the context. Initialization is by default lazy and automatic, but initialization can be forced manually if needed.

Example for custom configuration using the context builder:

 OutputStream myOut = new BufferedOutputStream()
 Context context = Context.newBuilder("js", "R")
                          .out(myOut)
                          .option("js.Strict", "true")
                          .allowAllAccess(true)
                          .build();
 context.eval("js", "42");
 context.eval("R", "42");
 context.close();
 

In this example:

• At first, we create a new context and specify permitted languages as parameters.
• Secondly, we set the standard output stream to be used for the context.
• Then, we specify an option for JavaScript language only, by structuring the option key with the language id followed by the option name;
• With Context.Builder.allowAllAccess(boolean) we grant a new context instance with the same access privileges as the host virtual machine.
• Next, we evaluate the expression "42" with language "js", which is the language identifier for JavaScript. Since this is the first time we access JavaScript, it first gets initialized as well.
• Similarly to the previous line, the R language expression gets evaluated.
• Finally, we close the context, since it is no longer needed, to free all allocated resources. Contexts are also AutoCloseable for use with the Java try-with-resources statement.

Bindings

The symbols of the top-most scope of a language can be accessed using the language bindings. Each language provides its own bindings object for a context. The bindings object may be used to read, modify, insert and delete members in the top-most scope of the language. Certain languages may not allow write access to the bindings object. See Context.getBindings(String) for details.

A context instance also provides access to the polyglot bindings. The polyglot bindings are shared between languages and may be used to exchange values. See Context.getPolyglotBindings() for details.

Examples using language bindings from JavaScript:

 Context context = Context.create("js");
 Value jsBindings = context.getBindings("js")

 jsBindings.putMember("foo", 42);
 assert context.eval("js", "foo").asInt() == 42;

 context.eval("js", "var bar = 42");
 assert jsBindings.getMember("bar").asInt() == 42;

 assert jsBindings.getMember("Math")
                  .getMember("abs")
                  .execute(-42)
                  .asInt() == 42;
 context.close();
 

In this example:

• We create a new context with JavaScript as the only permitted language.
• Next, we load the JavaScript bindings object and assign it to a local variable jsBindings.
• Then, we insert a new member foo into to the bindings object and verify that the object is accessible within the language by reading from a global symbol with the same name.
• After that, we declare a new global variable in JavaScript and verify that it is accessible as member of the language bindings object.
• Next, we access we access a JavaScript builtin named Math.abs symbol and execute it with -42. This result is asserted to be 42.
• Finally, we close the context to free all allocated resources.

Host Interoperability

It is often necessary to interact with values of the host runtime and Graal guest languages. Such objects are referred to as host objects. Every Java value that is passed to a Graal language is interpreted according to the specification described in Context.asValue(Object). Also see Value.as(Class) for further details.

By default only public classes, methods, and fields that are annotated with @HostAccess.Export are accessible to the guest language. This policy can be customized using Context.Builder.allowHostAccess(HostAccess) when constructing the context.

Example using a Java object from JavaScript:

 public class JavaRecord {
     @HostAccess.Export public int x;

     @HostAccess.Export
     public String name() {
         return "foo";
     }
 }
 Context context = Context.create();

 JavaRecord record = new JavaRecord();
 context.getBindings("js").putMember("javaRecord", record);

 context.eval("js", "javaRecord.x = 42");
 assert record.x == 42;

 context.eval("js", "javaRecord.name()").asString().equals("foo");
 

Error Handling

Program execution may fail when executing a guest language code or when accessing guest language object. Almost all methods in the Context and Value API throw a PolyglotException in case an error occurs. See PolyglotException for further details on error handling.

Isolation

Each context is by default isolated from all other instances with respect to both language evaluation semantics and resource consumption. By default, a new context instance has no access to host resources, like threads, files or loading new host classes. To allow access to such resources either the individual access right must be granted or all access must be set to true.

Contexts can be configured to share certain system resources like ASTs or optimized code by specifying a single underlying engine. See Engine for more details about code sharing.

Context can be configured to allow value sharing between multiple contexts (allowed by default). See Context.Builder.allowValueSharing(boolean) for details.

Proxies

The proxy interfaces allow to mimic guest language objects, arrays, executables, primitives and native objects in Graal languages. Every Graal language will treat proxy instances like objects of that particular language. Multiple proxy interfaces can be implemented at the same time. For example, it is useful to provide proxy values that are objects with members and arrays at the same time.

Thread-Safety

It is safe to use a context instance from a single thread. It is also safe to use it with multiple threads if they do not access the context at the same time. Whether a single context instance may be used from multiple threads at the same time depends on if all initialized languages support it. If initialized languages support multi-threading, then the context instance may be used from multiple threads at the same time. If a context is used from multiple threads and the language does not fit, then an IllegalStateException is thrown by the accessing method.

Meta-data from the context's underlying engine can be retrieved safely by any thread at any time.

A context may be closed from any thread, but only if the context is not currently executing code. If the context is currently executing some code, a different thread may kill the running execution and close the context using Context.close(boolean).

Context Exit

A context is exited naturally by calling the Context.close(boolean) method. A context may also be exited at the guest application request. There are two ways a guest language may exit.

• Soft exit. A guest language throws a special exception that causes the embedder thread to eventually throw a PolyglotException with PolyglotException.isExit() returning true and PolyglotException.getExitStatus() returning the exit status code specified by the guest application. The special exception does not influence other threads and does not trigger context close on its own. Closing the context is up to the embedder.
• Hard exit. A guest language uses a builtin command that unwinds all context threads and closes the context by force. Embedder threads also throw a PolyglotException with PolyglotException.isExit() returning true and PolyglotException.getExitStatus() returning the exit status code specified by the guest application. However, the context is closed automatically. The hard exit can be customized using Context.Builder.useSystemExit(boolean). If true, the context threads are unwound by calling System.exit(int) with the exit status parameter specified by the guest application. This operation terminates the whole host application.

Pre-Initialization

The context pre-initialization can be used to perform expensive builtin creation in the time of native compilation.

The context pre-initialization is enabled by setting the system property polyglot.image-build-time.PreinitializeContexts to a comma separated list of language ids which should be pre-initialized, for example: -Dpolyglot.image-build-time.PreinitializeContexts=js,python

See com.oracle.truffle.api.TruffleLanguage.patchContext(java.lang.Object, com.oracle.truffle.api.TruffleLanguage.Env) for details about pre-initialization for language implementers.

Since:

19.0

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	Context.Builder Builder class to construct Context instances.

Method Summary

Modifier and Type	Method and Description
Value	asValue(Object hostValue) Converts a host value to a polyglot value representation.
void	close() Closes this context and frees up potentially allocated native resources.
void	close(boolean cancelIfExecuting) Closes the context and frees up potentially allocated native resources.
static Context	create(String... permittedLanguages) Creates a context with default configuration.
void	enter() Explicitly enters the context on the current thread.
boolean	equals(Object obj)
Value	eval(Source source) Evaluates a source object by using the language specified in the source.
Value	eval(String languageId, CharSequence source) Evaluates a guest language code literal, using a provided language id.
Value	getBindings(String languageId) Returns a value that represents the top-most bindings of a language.
static Context	getCurrent() Returns the currently entered polyglot context.
Engine	getEngine() Provides access to meta-data about the underlying Graal engine.
Value	getPolyglotBindings() Returns polyglot bindings that may be used to exchange symbols between the host and guest languages.
int	hashCode()
boolean	initialize(String languageId) Forces the initialization of a language.
void	interrupt(Duration timeout) Use this method to interrupt this context.
void	leave() Explicitly leaves the context on the current thread.
static Context.Builder	newBuilder(String... permittedLanguages) Creates a builder for constructing a context with custom configuration.
Value	parse(Source source) Parses but does not evaluate a given source by using the language specified in the source and returns a value that can be executed.
Value	parse(String languageId, CharSequence source) Parses but does not evaluate a guest language code literal using a provided language id and character sequence and returns a value that can be executed.
void	resetLimits() Resets all accumulators of resource limits for the associated context to zero.
void	safepoint() Polls safepoints events and executes them for the current thread.

Methods inherited from class java.lang.Object

clone, getClass, notify, notifyAll, toString, wait, wait, wait

Method Detail

getEngine

public Engine getEngine()

Provides access to meta-data about the underlying Graal engine.

Returns:

Graal Engine being used by this context

Since:

19.0

eval

public Value eval(Source source)

Evaluates a source object by using the language specified in the source. The result is accessible as value and never returns null. The first time a source is evaluated, it will be parsed. Consecutive invocations of eval with the same source will only execute the already parsed code.

Basic Example:

 try (Context context = Context.create()) {
     Source source = Source.newBuilder("js", "42", "mysource.js").build();
     Value result = context.eval(source);
     assert result.asInt() == 42;
 }
 

Parameters:

source - a source object to evaluate

Returns:

the evaluation result. The returned instance is never null, but the result might represent a null value.

Throws:

PolyglotException - in case the guest language code parsing or evaluation failed.

IllegalStateException - if the context is already closed and the current thread is not allowed to access it.

IllegalArgumentException - if the language of the given source is not installed or the MIME type is not supported with the language.

Since:

19.0

eval

public Value eval(String languageId,
                  CharSequence source)

Evaluates a guest language code literal, using a provided language id. The result is accessible as value and never returns null. The provided CharSequence must represent an immutable String.

Basic Example:

 try (Context context = Context.create()) {
     Value result = context.eval("js", "42");
     assert result.asInt() == 42;
 }
 

Returns:

the evaluation result. The returned instance is never null, but the result might represent a null value.

Throws:

PolyglotException - in case the guest language code parsing or evaluation failed.

IllegalArgumentException - if the language does not exist or is not accessible.

IllegalStateException - if the context is already closed and the current thread is not allowed to access it, or if the given language is not installed.

Since:

19.0

parse

public Value parse(Source source)
            throws PolyglotException

Parses but does not evaluate a given source by using the language specified in the source and returns a value that can be executed. If a parsing fails, e.g. due to a syntax error in the source, then a PolyglotException will be thrown. In case of a syntax error the PolyglotException.isSyntaxError() will return true. There is no guarantee that only syntax errors will be thrown by this method. Any other guest language exception might be thrown. If the validation succeeds then the method completes without throwing an exception.

The result value only supports an empty set of arguments to execute. If executed repeatedly then the source is evaluated multiple times. Interactive sources will print their result for each execution of the parsing result to the output stream.

If the parsing succeeds and the source is cached then the result will automatically be reused for consecutive calls to Context.parse(Source) or Context.eval(Source). If the validation should be performed for each invocation or the result should not be remembered then cached can be set to false. By default sources are cached.

Basic Example:

 try (Context context = Context.create()) {
     Source source = Source.create("js", "42");
     Value value;
     try {
         value = context.parse(source);
         // parsing succeeded
     } catch (PolyglotException e) {
         if (e.isSyntaxError()) {
             SourceSection location = e.getSourceLocation();
             // syntax error detected at location
         } else {
             // other guest error detected
         }
         throw e;
     }
     // evaluate the parsed script
     value.execute();
 }
 

Parameters:

source - a source object to parse

Throws:

PolyglotException - in case the guest language code parsing or validation failed.

IllegalArgumentException - if the language does not exist or is not accessible.

IllegalStateException - if the context is already closed and the current thread is not allowed to access it, or if the given language is not installed.

Since:

20.2

parse

public Value parse(String languageId,
                   CharSequence source)

Parses but does not evaluate a guest language code literal using a provided language id and character sequence and returns a value that can be executed. The provided CharSequence must represent an immutable String. This method represents a short-hand for Context.parse(Source) .

The result value only supports an empty set of arguments to execute. If executed repeatedly then the source is evaluated multiple times. Interactive sources will print their result for each execution of the parsing result to the output stream.

 try (Context context = Context.create()) {
     Value value;
     try {
         value = context.parse("js", "42");
         // parsing succeeded
     } catch (PolyglotException e) {
         if (e.isSyntaxError()) {
             SourceSection location = e.getSourceLocation();
             // syntax error detected at location
         } else {
             // other guest error detected
         }
         throw e;
     }
     // evaluate the parsed script
     value.execute();
 }
 

Throws:

PolyglotException - in case the guest language code parsing or evaluation failed.

IllegalArgumentException - if the language does not exist or is not accessible.

IllegalStateException - if the context is already closed and the current thread is not allowed to access it, or if the given language is not installed.

Since:

20.2

getPolyglotBindings

public Value getPolyglotBindings()

Returns polyglot bindings that may be used to exchange symbols between the host and guest languages. All languages have unrestricted access to the polyglot bindings. The returned bindings object always has members and its members are readable, writable and removable.

Guest languages may put and get members through language specific APIs. For example, in JavaScript, symbols of the polyglot bindings can be accessed using Polyglot.import("name") and set using Polyglot.export("name", value). Please see the individual language reference on how to access these symbols.

Throws:

IllegalStateException - if context is already closed.

Since:

19.0

getBindings

public Value getBindings(String languageId)

Returns a value that represents the top-most bindings of a language. The top most bindings of the language returns a member for a symbol in the scope. Languages may allow modifications of members of the returned bindings object at the language's discretion. If the language has not been initialized yet, it will be initialized when the bindings are requested.

Throws:

IllegalArgumentException - if the language does not exist or is not accessible.

IllegalStateException - if the context is already closed.

PolyglotException - in case the lazy initialization failed due to a guest language error.

Since:

19.0

initialize

public boolean initialize(String languageId)

Forces the initialization of a language. It is not necessary to explicitly initialize a language, it will be initialized the first time it is used.

Parameters:

languageId - the identifier of the language to initialize.

Returns:

true if the language was initialized. Returns false if it was already initialized.

Throws:

PolyglotException - in case the initialization failed due to a guest language error.

IllegalArgumentException - if the language does not exist or is not accessible.

IllegalStateException - if the context is already closed.

Since:

19.0

resetLimits

public void resetLimits()

Resets all accumulators of resource limits for the associated context to zero.

Since:

19.3

asValue

public Value asValue(Object hostValue)

Converts a host value to a polyglot value representation. This conversion is applied implicitly whenever execution or instantiation arguments are provided, members and array elements are set or when a value is returned by a polyglot proxy. It is not required nor efficient to explicitly convert to polyglot values before performing these operations. This method is useful to convert a mapped host value back to a polyglot value while preserving the identity.

When a host value is converted to a polyglot value the following rules apply:

1. If the hostValue is null, then it will be interpreted as polyglot null.
2. If the hostValue is already a polyglot value, then it will be cast to Value.
3. If the hostValue is an instance of Byte, Short, Integer, Long, Float or Double, then it will be interpreted as polyglot number. Other subclasses of Number will be interpreted as host object (see later).
4. If the hostValue is an instance of Character or String, then it will be interpreted as polyglot string.
5. If the hostValue is an instance of Boolean, then it will be interpreted as polyglot boolean.
6. If the hostValue is an instance of Instant, LocalTime, ZonedDateTime, Date but not Date or Time then it will be interpreted as polyglot time.
7. If the hostValue is an instance of Instant, LocalDate, ZonedDateTime, Date but not Time or Date then it will be interpreted as polyglot date.
8. If the hostValue is an instance of ZoneId, Instant, ZonedDateTime, Date but not Time and Date then it will be interpreted as polyglot time zone.
9. If the hostValue is an instance of ZonedDateTime, Instant, ZonedDateTime, Date but not Time and Date then it will be interpreted as polyglot instant.
10. If the hostValue is an instance of Duration then it will be interpreted as polyglot duration.
11. If the hostValue is a polyglot proxy, then it will be interpreted according to the behavior specified by the proxy. See the javadoc of the proxy subclass for further details.
12. If the hostValue is a non-primitive mapped Java value, then the original value will be restored. For example, if a guest language object was mapped to Map, then the original object identity will be preserved when converting back to a polyglot value.
13. Any other hostValue will be interpreted as host object. Host objects expose all their public java fields and methods as members. In addition, Java arrays, subtypes of List and Map.Entry will be interpreted as a value with array elements. The subtypes of Iterable will be interpreted as a value with Value.hasIterator() iterator}. The subtypes of Iterator will be interpreted as an iterator value. The subtypes of Map will be interpreted as a value with Value.hasHashEntries() hash entries}. And single method interfaces annotated with FunctionalInterface are executable directly. Java Class instances are interpreted as instantiable, but they do not expose Class methods as members.

Basic Examples: The following assertion statements always hold:

 Context context = Context.create();
 assert context.asValue(null).isNull();
 assert context.asValue(42).isNumber();
 assert context.asValue("42").isString();
 assert context.asValue('c').isString();
 assert context.asValue(new String[0]).hasArrayElements();
 assert context.asValue(new ArrayList<>()).isHostObject();
 assert context.asValue(new ArrayList<>()).hasArrayElements();
 assert context.asValue((Supplier) () -> 42).execute().asInt() == 42;
 

Mapping to Java methods and fields

When Java host objects are passed to guest languages, their public methods and fields are provided as members. Methods and fields are grouped by name, so only one member is exposed for each name.

Class objects have a member named static referring to the class's companion object containing the static methods of the class. Likewise, the companion object has a member named class that points back to the class object.

When an argument value needs to be mapped to match a required Java method parameter type, then the semantics of host value mapping is used. The result of the mapping is equivalent of calling Value.as(Class) with the parameter type. Therefore, a ClassCastException or NullPointerException is thrown if a parameter value cannot be cast to the required parameter type.

Overloaded java methods are selected based on the provided arguments. In case multiple mapped Java methods with the same name are applicable for executions or instantiations, then the method with the most concrete method with applicable arguments will be called.

The following parameter type hierarchy is used for a method resolution. Left-most parameter types are prioritized over types to their right.

• Boolean values: boolean, Boolean, Object
• String values: char, Character, String, CharSequence, Object
• Number values: byte, Byte, short, Short, int, Integer, long, Long, float, Float, double, Double, Number, Object
• Other values are resolved based on their Java type hierarchy.

If there are multiple concrete methods or too many arguments provided, then an illegal argument type error will be raised.

Advanced Example: This example first creates a new instance of the Java class Record and inspects it using the polyglot value API. Later, a host value is converted to a polyglot value using JavaScript guest language.

In the following examples all assertions hold.

 class JavaRecord {
   public int x = 42;
   public double y = 42.0;
   public String name() {
     return "foo";
   }
 }
 Context context = Context.create();
 Value record = context.asValue(new JavaRecord());
 assert record.getMember("x").asInt() == 42;
 assert record.getMember("y").asDouble() == 42.0d;
 assert record.getMember("name").execute().asString().equals("foo");

 assert context.eval("js", "(function(record) record.x)")
               .execute(record).asInt() == 42;
 assert context.eval("js", "(function(record) record.y)")
               .execute(record).asDouble() == 42.0d;
 assert context.eval("js", "(function(record) record.name())")
               .execute(record).asString().equals("foo");
 

Parameters:

hostValue - the host value to convert to a polyglot value.

Returns:

the polyglot value.

Throws:

IllegalStateException - if the context is already closed

Since:

19.0

See Also:

Value.as(Class)

enter

public void enter()

Explicitly enters the context on the current thread. A context needs to be entered and left for any operation to be performed. For example, before and after invoking the execute method. This can be inefficient if a very high number of simple operations needs to be performed. By entering and leaving once explicitly, the overhead for entering/leaving the context for each operation can be eliminated. Contexts can be entered multiple times on the same thread.

Throws:

IllegalStateException - if the context is already closed.

PolyglotException - if a language has denied execution on the current thread.

Since:

19.0

See Also:

leave a context.

equals

public boolean equals(Object obj)

Overrides:

equals in class Object

Since:

19.0

hashCode

public int hashCode()

Overrides:

hashCode in class Object

Since:

19.0

leave

public void leave()

Explicitly leaves the context on the current thread. The context must be entered before calling this method.

Throws:

IllegalStateException - if the context is already closed or if the context was not entered on the current thread.

Since:

19.0

See Also:

enter a context.

close

public void close(boolean cancelIfExecuting)

Closes the context and frees up potentially allocated native resources. A context cannot free all native resources allocated automatically. For this reason it is necessary to close contexts after use. If a context is cancelled, then the executing thread will throw a PolyglotException. The exception indicates that it was cancelled. Please note, canceling a single context can negatively affect the performance of other executing contexts constructed with the same engine.

If internal errors occur during context closing, then they are printed to the configured error output stream. If a context was closed, then its methods will throw an IllegalStateException when invoked. If an attempt to close a context was successful, then consecutive calls to close have no effect.

For convenience, before the actual closing process begins, the close method leaves the context on the current thread, if it was entered explicitly.

Parameters:

cancelIfExecuting - if true then currently executing contexts will be cancelled, else an IllegalStateException is thrown.

Throws:

PolyglotException - in case the close failed due to a guest language error, or, if cancelIfExecuting is false, the exception is also thrown when the context was cancelled or the context was exited at request of the guest application.

IllegalStateException - if the context is still running and cancelIfExecuting is false

Since:

19.0

See Also:

close an engine.

close

public void close()

Closes this context and frees up potentially allocated native resources. A context may not free all native resources allocated automatically. For this reason it is recommended to close contexts after use. If the context is currently being executed on another thread, then an IllegalStateException is thrown. To close concurrently executing contexts see Context.close(boolean).

If internal errors occur during the context closure, then they are printed to the configured error output stream. If a context was closed, then its methods will throw an IllegalStateException, when invoked. If an attempt to close a context was successful, then consecutive calls to close have no effect.

For convenience, before the actual closing process begins, the close method leaves the context on the current thread, if it was entered explicitly.

Specified by:

close in interface AutoCloseable

Throws:

PolyglotException - in case the close failed due to a guest language error, or the context was cancelled or the context was exited at request of the guest application.

IllegalStateException - if the context is currently executing on another thread.

Since:

19.0

See Also:

close an engine.

interrupt

public void interrupt(Duration timeout)
               throws TimeoutException

Use this method to interrupt this context. The interruption is non-destructive meaning the context is still usable after this method finishes. Please note that guest finally blocks are executed during interrupt. A context thread may not be interruptiple if it uses non-interruptible waiting or executes non-interruptible host code. This method may be used as a "soft cancel", meaning that it can be used before close(true) is executed.

Parameters:

timeout - specifies the duration the interrupt method will wait for the active threads of the context to be finished. Setting the duration to 0 means wait indefinitely.

Throws:

IllegalStateException - in case the context is entered in the current thread.

TimeoutException - in case the interrupt was not successful, i.e., not all threads were finished within the specified time limit.

Since:

20.3

safepoint

public void safepoint()

Polls safepoints events and executes them for the current thread. This allows guest languages to run actions between long running host method calls. Polyglot embeddings that rely on cancellation should call this method whenev a potentially long running host operation is executed. For example, iterating an unbounded array. Guest language code and operations automatically poll safepoints regularly.

In this example we allow interruption and cancellation to stop the processing of our event queue.

 class EventProcessor {

   List