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# 4.1. `py4j.java_gateway` — Py4J Main API

The `py4j.java_gateway` module defines most of the classes that are needed to use Py4J. Py4J users are expected to only use explicitly `JavaGateway` and optionally, `GatewayParameters`, `CallbackServerParameters`, `java_import`, `get_field`, `get_method`, `launch_gateway`, and `is_instance_of`. The other module members are documented to support the extension of Py4J.

## 4.1.1. JavaGateway

class `py4j.java_gateway.``JavaGateway`(gateway_client=None, auto_field=False, python_proxy_port=25334, start_callback_server=False, auto_convert=False, eager_load=False, gateway_parameters=None, callback_server_parameters=None)
  A JavaGateway is the main interaction point between a Python VM and
    a JVM.
  - A JavaGateway instance is connected to a Gateway instance on the Java side.
  - The entry_point field of a JavaGateway instance is connected to the Gateway.entryPoint instance on the Java side.
  - The java_gateway_server field of a JavaGateway instance is connected to the GatewayServer instance on the Java side.
  - The jvm field of JavaGateway enables user to access classes, static members (fields and methods) and call constructors.

  Methods that are not defined by JavaGateway are always redirected to entry_point. For example, `gateway.doThat()` is equivalent to `gateway.entry_point.doThat()`. This is a trade-off between convenience and potential confusion.

  Parameters: | gateway_parameters – An instance of GatewayParameters used to configure the various options of the gateway. callback_server_parameters – An instance of CallbackServerParameters used to configure various options of the gateway server. Must be provided to start a gateway server. Otherwise, callbacks won”t be available.

  `close`(keep_callback_server=False)
    Closes all gateway connections. A connection will be reopened if
      necessary (e.g., if a `JavaMethod` is called).

    Parameters: | keep_callback_server – if True, the callback server is not shut down.

  `detach`(java_object)

    Makes the Java Gateway dereference this object.

    The equivalent of this method is called when a JavaObject instance is garbage collected on the Python side. This method, or gc.collect() should still be invoked when memory is limited or when too many objects are created on the Java side.

    Parameters: | java_object – The JavaObject instance to dereference (free) on the Java side.

  `get_callback_server`()
  `help`(var, pattern=None, short_name=True, display=True)

    Displays a help page about a class or an object.

    Parameters: | var – JavaObject, JavaClass or JavaMember for which a help page will be generated. pattern – Star-pattern used to filter the members. For example “get*Foo” may return getMyFoo, getFoo, getFooBar, but not bargetFoo. The pattern is matched against the entire signature. To match only the name of a method, use “methodName(*”. short_name – If True, only the simple name of the parameter types and return types will be displayed. If False, the fully qualified name of the types will be displayed. display – If True, the help page is displayed in an interactive page similar to the help command in Python. If False, the page is returned as a string.

  classmethod `launch_gateway`(port=0, jarpath=u'', classpath=u'', javaopts=[], die_on_exit=False, redirect_stdout=None, redirect_stderr=None, daemonize_redirect=True)

    Launch a Gateway in a new Java process and create a default `JavaGateway` to connect to it.

    See `launch_gateway` for more information about this function.

    Parameters: | port – the port to launch the Java Gateway on. If no port is specified then an ephemeral port is used. jarpath – the path to the Py4J jar. Only necessary if the jar was installed at a non-standard location or if Python is using a different sys.prefix than the one that Py4J was installed under. classpath – the classpath used to launch the Java Gateway. javaopts – an array of extra options to pass to Java (the classpath should be specified using the classpath parameter, not javaopts.) die_on_exit – if True, the Java gateway process will die when this Python process exits or is killed. redirect_stdout – where to redirect the JVM stdout. If None (default) stdout is redirected to os.devnull. Otherwise accepts a file descriptor, a queue, or a deque. Will send one line at a time to these objects. redirect_stderr – where to redirect the JVM stdout. If None (default) stderr is redirected to os.devnull. Otherwise accepts a file descriptor, a queue, or a deque. Will send one line at a time to these objects. daemonize_redirect – if True, the consumer threads will be daemonized and will not prevent the main Python process from exiting. This means the file descriptors (stderr, stdout, redirect_stderr, redirect_stdout) might not be properly closed. This is not usually a problem, but in case of errors related to file descriptors, set this flag to False.
    Return type: | a `JavaGateway` connected to the Gateway server.

  `new_array`(java_class, *dimensions)

    Creates a Java array of type java_class of dimensions

    Parameters: | java_class – The `JavaClass` instance representing the type of the array. dimensions – A list of dimensions of the array. For example [1,2] would produce an array[1][2].
    Return type: | A `JavaArray` instance.

  `new_jvm_view`(name=u'custom jvm')

    Creates a new JVM view with its own imports. A JVM view ensures that the import made in one view does not conflict with the import of another view.

    Generally, each Python module should have its own view (to replicate Java behavior).

    Parameters: | name – Optional name of the jvm view. Does not need to be unique, i.e., two distinct views can have the same name (internally, they will have a distinct id).
    Return type: | A JVMView instance (same class as the gateway.jvm instance).

  `restart_callback_server`()

    Shuts down the callback server (if started) and restarts a new one.

  `set_gateway_client`(gateway_client)

    Sets the gateway client for this JavaGateway. This sets the appropriate gateway_property and resets the main jvm view (self.jvm).

    This is for advanced usage only. And should only be set before the gateway is loaded.

  `shutdown`(raise_exception=False)
    Shuts down the `GatewayClient` and the
      `CallbackServer`.

    Parameters: | raise_exception – If True, raise an exception if an error occurs while shutting down (very likely with sockets).

  `shutdown_callback_server`(raise_exception=False)
    Shuts down the
      `CallbackServer`.

    Parameters: | raise_exception – If True, raise an exception if an error occurs while shutting down (very likely with sockets).

  `start_callback_server`(callback_server_parameters=None)

    Starts the callback server.

    Parameters: | callback_server_parameters – parameters to use to start the server. If not provided, it will use the gateway callback server parameters.
    Return type: | Returns True if the server was started by this call or False if it was already started (you cannot have more than one started callback server).

### 4.1.1.1. Examples

Using the `jvm` property:

    >>> gateway = JavaGateway()
    >>> jvm = gateway.jvm
    >>> l = jvm.java.util.ArrayList()
    >>> l.append(10)
    >>> l.append(1)
    >>> jvm.java.util.Collections.sort(l)
    >>> l
    [1, 10]
    >>> l.append(5)
    >>> l.sort()
    >>> l
    [1, 5, 10]

Using `auto_field`:

First we declare a class that has a field AND a method called member:

    package py4j.examples;
    public class ExampleWithField {
        public int member = 1;
        public String member() {
            return "Hello World";
        }
    }

Then we play with the class using the two possible values of auto_field:

    >>> java_gateway = JavaGateway() # auto_field = False
    >>> example = java_gateway.jvm.py4j.examples.ExampleWithField()
    >>> example.member()
    u'Hello World'
    >>> get_field(example,'member')
    1
    >>> java_gateway2 = JavaGateway(GatewayParameters(auto_field=True))
    >>> example2 = java_gateway2.jvm.py4j.examples.ExampleWithField()
    >>> example2.member
    1
    >>> get_method(example2,'member')()
    u'Hello World'

## 4.1.2. GatewayParameters

class `py4j.java_gateway.``GatewayParameters`(address=u'127.0.0.1', port=25333, auto_field=False, auto_close=True, auto_convert=False, eager_load=False)

  Wrapper class that contains all parameters that can be passed to configure a JavaGateway

  Parameters: | address – the address to which the client will request a connection port – the port to which the client will request a connection. Default is 25333. auto_field – if False, each object accessed through this gateway won”t try to lookup fields (they will be accessible only by calling get_field). If True, fields will be automatically looked up, possibly hiding methods of the same name and making method calls less efficient. auto_close – if True, the connections created by the client close the socket when they are garbage collected. auto_convert – if True, try to automatically convert Python objects like sequences and maps to Java Objects. Default value is False to improve performance and because it is still possible to explicitly perform this conversion. eager_load – if True, the gateway tries to connect to the JVM by calling System.currentTimeMillis. If the gateway cannot connect to the JVM, it shuts down itself and raises an exception.

## 4.1.3. CallbackServerParameters

class `py4j.java_gateway.``CallbackServerParameters`(address=u'127.0.0.1', port=25334, daemonize=False, daemonize_connections=False, eager_load=True)

  Wrapper class that contains all parameters that can be passed to configure a CallbackServer

  Parameters: | address – the address to which the client will request a connection port – the port to which the client will request a connection. Default is 25333. daemonize – If True, will set the daemon property of the server thread to True. The callback server will exit automatically if all the other threads exit. daemonize_connections – If True, callback server connections are executed in daemonized threads and will not block the exit of a program if non daemonized threads are finished. eager_load – If True, the callback server is automatically started when the JavaGateway is created.

## 4.1.4. GatewayClient

class `py4j.java_gateway.``GatewayClient`(address=u'127.0.0.1', port=25333, auto_close=True, gateway_property=None)

  Responsible for managing connections to the JavaGateway.

  This implementation is thread-safe and connections are created on-demand. This means that Py4J-Python can be accessed by multiple threads and messages are sent to and processed concurrently by the Java Gateway.

  When creating a custom `JavaGateway`, it is recommended to pass an instance of `GatewayClient` instead of a `GatewayConnection`: both have the same interface, but the client supports multiple threads and connections, which is essential when using callbacks.

  Parameters: | address – the address to which the client will request a connection port – the port to which the client will request a connection. Default is 25333. auto_close – if True, the connections created by the client close the socket when they are garbage collected. gateway_property – used to keep gateway preferences without a cycle with the gateway

  `close`()

    Closes all currently opened connections.

    This operation is not thread safe and is only a best effort strategy to close active connections.

    All connections are guaranteed to be closed only if no other thread is accessing the client and no call is pending.

  `send_command`(command, retry=True)
    Sends a command to the JVM. This method is not intended to be
      called directly by Py4J users. It is usually called by `JavaMember` instances.

    Parameters: | command – the string command to send to the JVM. The command must follow the Py4J protocol. retry – if True, the GatewayClient tries to resend a message if it fails.
    Return type: | the string answer received from the JVM. The answer follows the Py4J protocol.

  `shutdown_gateway`()

    Sends a shutdown command to the gateway. This will close the gateway server: all active connections will be closed. This may be useful if the lifecycle of the Java program must be tied to the Python program.

## 4.1.5. GatewayConnection

class `py4j.java_gateway.``GatewayConnection`(address=u'127.0.0.1', port=25333, auto_close=True, gateway_property=None)

  Default gateway connection (socket based) responsible for communicating with the Java Virtual Machine.

  Parameters: | address – the address to which the connection will be established port – the port to which the connection will be established. Default is 25333. auto_close – if True, the connection closes the socket when it is garbage collected. gateway_property – contains gateway preferences to avoid a cycle with gateway

  `close`()

    Closes the connection by closing the socket.

  `send_command`(command)
    Sends a command to the JVM. This method is not intended to be
      called directly by Py4J users: it is usually called by JavaMember instances.

    Parameters: | command – the string command to send to the JVM. The command must follow the Py4J protocol.
    Return type: | the string answer received from the JVM. The answer follows the Py4J protocol.

  `shutdown_gateway`()

    Sends a shutdown command to the gateway. This will close the gateway server: all active connections will be closed. This may be useful if the lifecycle of the Java program must be tied to the Python program.

  `start`()

    Starts the connection by connecting to the address and the port

## 4.1.6. JVMView

class `py4j.java_gateway.``JVMView`(gateway_client, jvm_name, id=None, jvm_object=None)

  A JVMView allows access to the Java Virtual Machine of a JavaGateway.

  This can be used to reference static members (fields and methods) and to call constructors.

## 4.1.7. JavaObject

Represents a Java object from which you can call methods or access fields.

## 4.1.8. JavaMember

Represents a member (i.e., method) of a `JavaObject`. For now, only methods are supported. Fields are retrieved directly and are not contained in a JavaMember.

## 4.1.9. JavaClass

A JavaClass represents a Java Class from which static members can be retrieved. JavaClass instances are also needed to initialize an array.

Usually, JavaClass are not initialized using their constructor, but they are created while accessing the jvm property of a gateway, e.g., gateway.jvm.java.lang.String.

## 4.1.10. JavaPackage

class `py4j.java_gateway.``JavaPackage`(fqn, gateway_client, jvm_id=None)

  A JavaPackage represents part of a Java package from which Java classes can be accessed.

  Usually, JavaPackage are not initialized using their constructor, but they are created while accessing the jvm property of a gateway, e.g., gateway.jvm.java.lang.

## 4.1.11. PythonProxyPool

class `py4j.java_gateway.``PythonProxyPool`

  A PythonProxyPool manages proxies that are passed to the Java side. A proxy is a Python class that implements a Java interface.

  A proxy has an internal class named Java with a member named implements which is a list of fully qualified names (string) of the implemented interfaces.

  The PythonProxyPool implements a subset of the dict interface: pool[id], del(pool[id]), pool.put(proxy), pool.clear(), id in pool, len(pool).

  The PythonProxyPool is thread-safe.

  `clear`()
  `put`(object)

    Adds a proxy to the pool.

    Parameters: | object – The proxy to add to the pool.
    Return type: | A unique identifier associated with the object.

## 4.1.12. CallbackServer

class `py4j.java_gateway.``CallbackServer`(pool, gateway_client, port=25334, address=u'127.0.0.1', callback_server_parameters=None)

  The CallbackServer is responsible for receiving call back connection requests from the JVM. Usually connections are reused on the Java side, but there is at least one connection per concurrent thread.

  Parameters: | pool – the pool responsible of tracking Python objects passed to the Java side. gateway_client – the gateway client used to call Java objects. callback_server_parameters – An instance of CallbackServerParameters used to configure various options of the callback server.

  `get_listening_address`()

    Returns the address on which the callback server is listening to. May be different than address if address was an alias (e.g., localhost).

  `get_listening_port`()

    Returns the port on which the callback server is listening to. Different than port when port is 0.

  `run`()

    Starts listening and accepting connection requests.

    This method is called when invoking CallbackServer.start(). A CallbackServer instance is created and started automatically when a `JavaGateway` instance is created.

  `shutdown`()

    Stops listening and accepting connection requests. All live connections are closed.

    This method can safely be called by another thread.

  `start`()

    Starts the CallbackServer. This method should be called by the client instead of run().

## 4.1.13. CallbackConnection

class `py4j.java_gateway.``CallbackConnection`(pool, input, socket_instance, gateway_client, callback_server_parameters)

  A CallbackConnection receives callbacks and garbage collection requests from the Java side.

  `run`()

## 4.1.14. Py4J Functions

The following functions get be used to import packages or to get a particular field or method when fields and methods in a Java class have the same name:

`py4j.java_gateway.``java_import`(jvm_view, import_str)

  Imports the package or class specified by import_str in the jvm view namespace.

  Parameters: | jvm_view – The jvm_view in which to import a class/package.
  Import_str: | The class (e.g., java.util.List) or the package (e.g., java.io.*) to import

`py4j.java_gateway.``launch_gateway`(port=0, jarpath=u'', classpath=u'', javaopts=[], die_on_exit=False, redirect_stdout=None, redirect_stderr=None, daemonize_redirect=True)

  Launch a Gateway in a new Java process.

  The redirect parameters accept file-like objects, Queue, or deque. When text lines are sent to the stdout or stderr of the child JVM, these lines are redirected to the file-like object (`write(line)`), the Queue (`put(line)`), or the deque (`appendleft(line)`).

  The text line will contain a newline character.

  Only text output is accepted on stdout and stderr. If you wish to communicate with the child JVM through bytes, you need to create your own helper function.

  Parameters: | port – the port to launch the Java Gateway on. If no port is specified then an ephemeral port is used. jarpath – the path to the Py4J jar. Only necessary if the jar was installed at a non-standard location or if Python is using a different sys.prefix than the one that Py4J was installed under. classpath – the classpath used to launch the Java Gateway. javaopts – an array of extra options to pass to Java (the classpath should be specified using the classpath parameter, not javaopts.) die_on_exit – if True, the Java gateway process will die when this Python process exits or is killed. redirect_stdout – where to redirect the JVM stdout. If None (default) stdout is redirected to os.devnull. Otherwise accepts a file descriptor, a queue, or a deque. Will send one line at a time to these objects. redirect_stderr – where to redirect the JVM stdout. If None (default) stderr is redirected to os.devnull. Otherwise accepts a file descriptor, a queue, or a deque. Will send one line at a time to these objects. daemonize_redirect – if True, the consumer threads will be daemonized and will not prevent the main Python process from exiting. This means the file descriptors (stderr, stdout, redirect_stderr, redirect_stdout) might not be properly closed. This is not usually a problem, but in case of errors related to file descriptors, set this flag to False.
  Return type: | the port number of the Gateway server.

`py4j.java_gateway.``get_field`(java_object, field_name)

  Retrieves the field named field_name from the java_object.

  This function is useful when auto_field=false in a gateway or Java object.

  Parameters: | java_object – the instance containing the field field_name – the name of the field to retrieve

`py4j.java_gateway.``set_field`(java_object, field_name, value)

  Sets the field named field_name of java_object to value.

  This function is the only way to set a field because the assignment operator in Python cannot be overloaded.

  Parameters: | java_object – the instance containing the field field_name – the name of the field to set value – the value to assign to the field

`py4j.java_gateway.``get_method`(java_object, method_name)

  Retrieves a reference to the method of an object.

  This function is useful when auto_field=true and an instance field has the same name as a method. The full signature of the method is not required: it is determined when the method is called.

  Parameters: | java_object – the instance containing the method method_name – the name of the method to retrieve

`py4j.java_gateway.``is_instance_of`(gateway, java_object, java_class)

  Indicates whether a java object is an instance of the provided java_class.

  Parameters: | gateway – the JavaGateway instance java_object – the JavaObject instance java_class – can be a string (fully qualified name), a JavaClass instance, or a JavaObject instance)

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