Ef cient I/O in Java

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is v102, and all applications were launched from slush No other processes were running The numbers achieved when the applications are loaded directly into the ash ROM are about 3 to 4 percent faster The performance of other TINI hardware implementations will of course vary depending largely on the processor clock rate System loading caused by processing network and other interrupts can also cause noticeable timing variations Serious variations can occur when measuring operations that require only a small amount of time for execution The variations can be caused by sudden changes in CPU load due to interrupts, from sources such as the Ethernet network controller, or just due to loss of execution because of either thread or process swapping For this reason, the test environment should be reasonably well controlled First, only one process should be actively executing Other live processes, such as init (typically the shell) and gc (the garbage collector), are ne as long as they are dormant (not actively being scheduled) To avoid high percentage errors in measurement when measuring relatively quick operations, perform the operation a number of times, typically in a loop, and measure the entire time Then divide the result by the number of loop iterations If the overall execution time is aimed at several seconds, then any error due to interrupt latency under normal loads will be negligibly small
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For most TINI applications, the rst priority is ef ciently moving data to and from system resources such as the serial port or Ethernet controller, as well as external application-speci c hardware TINI s runtime environment was written with this in mind The native I/O infrastructure was carefully coded so that data can be moved quickly from application provided buffers ( byte arrays) to system resources or attached circuitry The most important thing a Java application must do to take advantage of this infrastructure is to move data between the application and native drivers quickly This means moving data in reasonably large blocks as opposed to a single byte at a time The process of moving data to and from streams, or other I/O mechanisms, a byte at a time will be loosely termed byte-banging
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Like other Java platforms, much of the I/O on TINI is stream based, including network, le system, and serial port communication This means that moving data to and from an I/O resource usually boils down to invoking read and write methods on instances of subclasses of javaioInputStream and javaioOutputStream Ef cient I/O using streams can be achieved by utilizing these block read and write methods
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public int read(byte[] b, int off, int len) throws IOException public void write(byte[] b, int off, int len) throws IOException
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The default implementation of these methods provided in InputStream and is very inef cient The write method, for example, simply invokes the single byte write method iteratively len times to move len bytes of data to the underlying resource This makes sense because InputStream and OutputStream are not tied to any concrete I/O resource and therefore are unable to make any assumptions about the native interface provided for a speci c device or resource However, subclasses of InputStream and OutputStream override the read and write methods just shown The subclass s implementation maps directly to a native method call to a driver that takes the same parameters and performs the requested I/O The requested data transfer occurs at the expense of only one, rather than len, context switches from the Java application to the native runtime Listing 112 shows the worker thread of an echo server The echo server accepts connections from clients and creates an EchoWorker thread to manage the connection EchoWorker s constructor invokes the getInputStream and getOutputStream methods on the socket to get the lowest level, and therefore most ef cient, streams available for reading data from and writing data to the underlying connection These are actually instances of SocketInputStream and SocketOutputStream, which are private classes de ned in the javanet package The run method waits for receive data All data received is immediately transmitted (or echoed) back to the sender The run method will exit normally if the echo client closes the connection or abruptly if an IOException occurs during a network read or write operation Inbound data from the client is read by invoking the single-byte read method on the socket s input stream and written to the client using the single-byte write method on the socket s output stream
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