# Class Connection

java.lang.Object
com.sun.jdi.connect.spi.Connection

public abstract class Connection extends Object
A connection between a debugger and a target VM which it debugs.

A Connection represents a bi-directional communication channel between a debugger and a target VM. A Connection is created when `TransportService` establishes a connection and successfully handshakes with a target VM. A TransportService implementation provides a reliable JDWP packet transportation service and consequently a Connection provides a reliable flow of JDWP packets between the debugger and the target VM. A Connection is stream oriented, that is, the JDWP packets written to a connection are read by the target VM in the order in which they were written. Similarly packets written to a Connection by the target VM are read by the debugger in the order in which they were written.

A connection is either open or closed. It is open upon creation, and remains open until it is closed. Once closed, it remains closed, and any attempt to invoke an I/O operation upon it will cause a `ClosedConnectionException` to be thrown. A connection can be tested by invoking the `isOpen` method.

A Connection is safe for access by multiple concurrent threads, although at most one thread may be reading and at most one thread may be writing at any given time.

Since:
  1.5

## Constructor Summary

Constructors
Constructor
Description
`Connection()`
Constructor for subclasses to call.

## Method Summary

Modifier and Type
Method
Description
`abstract void`
`close()`
Closes this connection.
`abstract boolean`
`isOpen()`
Tells whether or not this connection is open.
`abstract byte[]`
`readPacket()`
Reads a packet from the target VM.
`abstract void`
`writePacket(byte[] pkt)`
Writes a packet to the target VM.

### Methods declared in class Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

## Constructor Details

### Connection

public Connection()
Constructor for subclasses to call.

## Method Details

### readPacket

public abstract byte[] readPacket() throws IOException
Reads a packet from the target VM.

Attempts to read a JDWP packet from the target VM. A read operation may block indefinitely and only returns when it reads all bytes of a packet, or in the case of a transport service that is based on a stream-oriented communication protocol, the end of stream is encountered.

Reading a packet does not do any integrity checking on the packet aside from a check that the length of the packet (as indicated by the value of the `length` field, the first four bytes of the packet) is 11 or more bytes. If the value of the `length` value is less then 11 then an `IOException` is thrown.

Returns a byte array of a length equal to the length of the received packet, or a byte array of length 0 when an end of stream is encountered. If end of stream is encountered after some, but not all bytes of a packet, are read then it is considered an I/O error and an `IOException` is thrown. The first byte of the packet is stored in element `0` of the byte array, the second in element `1`, and so on. The bytes in the byte array are laid out as per the JDWP specification. That is, all fields in the packet are in big endian order as per the JDWP specification.

This method may be invoked at any time. If another thread has already initiated a `readPacket` on this connection then the invocation of this method will block until the first operation is complete.

Returns:
  the packet read from the target VM
Throws:
  `ClosedConnectionException` - If the connection is closed, or another thread closes the connection while the readPacket is in progress.
  `IOException` - If the length of the packet (as indictaed by the first 4 bytes) is less than 11 bytes, or an I/O error occurs.

### writePacket

public abstract void writePacket(byte[] pkt) throws IOException
Writes a packet to the target VM.

Attempts to write, or send, a JDWP packet to the target VM. A write operation only returns after writing the entire packet to the target VM. Writing the entire packet does not mean the entire packet has been transmitted to the target VM but rather that all bytes have been written to the transport service. A transport service based on a TCP/IP connection may, for example, buffer some or all of the packet before transmission on the network.

The byte array provided to this method should be laid out as per the JDWP specification. That is, all fields in the packet are in big endian order. The first byte, that is element `pkt[0]`, is the first byte of the `length` field. `pkt[1]` is the second byte of the `length` field, and so on.

Writing a packet does not do any integrity checking on the packet aside from checking the packet length. Checking the packet length requires checking that the value of the `length` field (as indicated by the first four bytes of the packet) is 11 or greater. Consequently the length of the byte array provided to this method, that is `pkt.length`, must be 11 or more, and must be equal or greater than the value of the `length` field. If the length of the byte array is greater than the value of the `length` field then all bytes from element `pkt[length]` onwards are ignored. In other words, any additional bytes that follow the packet in the byte array are ignored and will not be transmitted to the target VM.

A write operation may block or may complete immediately. The exact circumstances when an operation blocks depends on the transport service. In the case of a TCP/IP connection to the target VM, the writePacket method may block if there is network congestion or there is insufficient space to buffer the packet in the underlying network system.

This method may be invoked at any time. If another thread has already initiated a write operation upon this Connection then a subsequent invocation of this method will block until the first operation is complete.

Parameters:
  `pkt` - The packet to write to the target VM.
Throws:
  `ClosedConnectionException` - If the connection is closed, or another thread closes the connection while the write operation is in progress.
  `IOException` - If an I/O error occurs.
  `IllegalArgumentException` - If the value of the `length` field is invalid, or the byte array is of insufficient length.

### close

public abstract void close() throws IOException
Closes this connection.

If the connection is already closed then invoking this method has no effect. After a connection is closed, any further attempt calls to `readPacket` or `writePacket` will throw a `ClosedConnectionException`.

Any thread currently blocked in an I/O operation (`readPacket` or `writePacket`) will throw a `ClosedConnectionException`).

This method may be invoked at any time. If some other thread has already invoked it, however, then another invocation will block until the first invocation is complete, after which it will return without effect.

Throws:
  `IOException` - If an I/O error occurs

### isOpen

public abstract boolean isOpen()
Tells whether or not this connection is open.

Returns:
  `true` if and only if this connection is open