Effective Java: Programming Language Guide in Java

Generator QR Code JIS X 0510 in Java Effective Java: Programming Language Guide
Effective Java: Programming Language Guide
Encode QR In Java
Using Barcode encoder for Java Control to generate, create QR Code JIS X 0510 image in Java applications.
expensive to initialize and may not be needed, but will be used intensively if it is needed This idiom is shown below:
Draw Barcode In Java
Using Barcode encoder for Java Control to generate, create bar code image in Java applications.
// The initialize-on-demand holder class idiom private static class FooHolder { static final Foo foo = new Foo(); } public static Foo getFoo() { return FooHolderfoo; }
Bar Code Decoder In Java
Using Barcode scanner for Java Control to read, scan read, scan image in Java applications.
The idiom takes advantage of the guarantee that a class will not be initialized until it is used [JLS, 1241] When the getFoo method is invoked for the first time, it reads the field FooHolderfoo, causing the FooHolder class to get initialized The beauty of this idiom is that the getFoo method is not synchronized and performs only a field access, so lazy initialization adds practically nothing to the cost of access The only shortcoming of the idiom is that it does not work for instance fields, only for static fields In summary, whenever multiple threads share mutable data, each thread that reads or writes the data must obtain a lock Do not let the guarantee of atomic reads and writes deter you from performing proper synchronization Without synchronization, there is no guarantee as to which, if any, of a thread's changes will be observed by another thread Liveness and safety failures can result from unsynchronized data access Such failures will be extremely difficult to reproduce They may be timing dependent and will be highly dependent on the details of the JVM implementation and the hardware on which it is running The use of the volatile modifier constitutes a viable alternative to ordinary synchronization under certain circumstances, but this is an advanced technique Furthermore, the extent of its applicability will not be known until the ongoing work on the memory model is complete
Paint QR Code ISO/IEC18004 In Visual C#.NET
Using Barcode generation for .NET framework Control to generate, create QR image in Visual Studio .NET applications.
Item 49: Avoid excessive synchronization
Paint QR Code 2d Barcode In .NET
Using Barcode generation for ASP.NET Control to generate, create QR Code image in ASP.NET applications.
Item 48 warns of the dangers of insufficient synchronization This item concerns the opposite problem Depending on the situation, excessive synchronization can cause reduced performance, deadlock, or even nondeterministic behavior To avoid the risk of deadlock, never cede control to the client within a synchronized method or block In other words, inside a synchronized region, do not invoke a public or protected method that is designed to be overridden (Such methods are typically abstract, but occasionally they have a concrete default implementation) From the perspective of the class containing the synchronized region, such a method is alien The class has no knowledge of what the method does and no control over it A client could provide an implementation of an alien method that creates another thread that calls back into the class The newly created thread might then try to acquire the same lock held by the original thread, which would cause the newly created thread to block If the method that created the thread waits for the thread to finish, deadlock results To make this concrete, consider the following class, which implements a work queue This class allows clients to enqueue work items for asynchronous processing The enqueue method may be invoked as often as necessary The constructor starts a background thread that removes items from the queue in the order they were enqueued and processes them by
Printing Denso QR Bar Code In VS .NET
Using Barcode generation for .NET Control to generate, create QR Code image in .NET framework applications.
Effective Java: Programming Language Guide
Denso QR Bar Code Encoder In VB.NET
Using Barcode creator for .NET Control to generate, create QR Code JIS X 0510 image in Visual Studio .NET applications.
invoking the processItem method When the work queue is no longer needed, the client invokes the stop method to ask the thread to terminate gracefully after completing any work item in progress
Creating UCC.EAN - 128 In Java
Using Barcode generation for Java Control to generate, create GTIN - 128 image in Java applications.
public abstract class WorkQueue { private final List queue = new LinkedList(); private boolean stopped = false; protected WorkQueue() { new WorkerThread()start(); } public final void enqueue(Object workItem) { synchronized (queue) { queueadd(workItem); queuenotify(); } } public final void stop() { synchronized (queue) { stopped = true; queuenotify(); } } protected abstract void processItem(Object workItem) throws InterruptedException; // Broken - invokes alien method from synchronized block! private class WorkerThread extends Thread { public void run() { while (true) { // Main loop synchronized (queue) { try { while (queueisEmpty() && !stopped) queuewait(); } catch (InterruptedException e) { return; } if (stopped) return; Object workItem = queueremove(0); try { processItem(workItem); // Lock held! } catch (InterruptedException e) { return; }
Creating Bar Code In Java
Using Barcode maker for Java Control to generate, create bar code image in Java applications.
} } } } }
Barcode Creator In Java
Using Barcode generator for Java Control to generate, create barcode image in Java applications.
To use this class, you must subclass it to provide an implementation of the abstract processItem method For example, the following subclass prints out each work item, printing no more than one item per second, no matter how frequently items are enqueued:
Code 3/9 Printer In Java
Using Barcode generation for Java Control to generate, create Code 39 Full ASCII image in Java applications.
Effective Java: Programming Language Guide class DisplayQueue extends WorkQueue { protected void processItem(Object workItem) throws InterruptedException { Systemoutprintln(workItem); Threadsleep(1000); } }
Making USS Code 128 In Java
Using Barcode creator for Java Control to generate, create Code-128 image in Java applications.
Because the WorkQueue class invokes the abstract processItem method from within a synchronized block, it is subject to deadlock The following subclass will cause it to deadlock by the means described above:
Draw RM4SCC In Java
Using Barcode drawer for Java Control to generate, create British Royal Mail 4-State Customer Code image in Java applications.
class DeadlockQueue extends WorkQueue { protected void processItem(final Object workItem) throws InterruptedException { // Create a new thread that returns workItem to queue Thread child = new Thread() { public void run() { enqueue(workItem); } }; childstart(); childjoin(); // Deadlock! } }
Barcode Recognizer In VS .NET
Using Barcode reader for VS .NET Control to read, scan read, scan image in Visual Studio .NET applications.
This example is contrived because there is no reason for the processItem method to create a background thread, but the problem is real Invoking externally provided methods from within synchronized blocks has caused many deadlocks in real systems such as GUI toolkits Luckily it is easy to fix the problem Simply move the method invocation outside of the synchronized block, as shown:
USS Code 39 Maker In Visual Basic .NET
Using Barcode creation for Visual Studio .NET Control to generate, create USS Code 39 image in .NET framework applications.
// Alien method outside synchronized block - "Open call" private class WorkerThread extends Thread { public void run() { while (true) { // Main loop Object workItem = null; synchronized (queue) { try { while (queueisEmpty() && !stopped) queuewait(); } catch (InterruptedException e) { return; } if (stopped) return; workItem = queueremove(0); } try { processItem(workItem); // No lock held } catch (InterruptedException e) { return; } } } }
Barcode Maker In .NET Framework
Using Barcode printer for Visual Studio .NET Control to generate, create barcode image in .NET applications.
Generate USS Code 39 In C#
Using Barcode generator for Visual Studio .NET Control to generate, create Code39 image in .NET framework applications.
Printing Code39 In .NET Framework
Using Barcode printer for VS .NET Control to generate, create Code 3 of 9 image in .NET framework applications.