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2013年10月18日 #

如果是Servlet , Action , Controller, 或则Filter , Listener , 拦截器等相关类时, 我们只需要获得ServletContext, 然后通过ServletContext.getRealPath("/")来获取当前应用在服务器上的物理地址

如果在类中取不到ServletContext时, 有两种方式可以做到

1) 利用Java的类加载机制 调用 XXX.class.getClassLoader().getResource(""); 方法来获取到ClassPath , 然后处理获得WebRoot目录,这种方式只能是该class在WebRoot/WEB-INF/classes下才能生效, 如果该class被打包到一个jar文件中, 则该方法失效。这时就应该用下面一种方式

2) spring框架的思路, 在WEB-INF/web.xml中 , 创建一个webAppRootKey的param, 指定一个值(默认为webapp.root)作为键值, 然后通过Listener , 或者Filter , 或者Servlet 执行String webAppRootKey = getServletContext().getRealPath("/"); 并将webAppRootKey对应的webapp.root 分别作为Key , Value写到System Properties系统属性中。之后在程序中通过System.getProperty("webapp.root")来获得WebRoot的物理路径


下面是一些得到classpath和当前类的绝对路径的一些方法。你可使用其中的一些方法来得到你需要的资源的绝对路径:

  1.DebitNoteAction.class.getResource("")

  得到的是当前类FileTest.class文件的URI目录。不包括自己!

  如:file:/D:/eclipse/springTest/WebRoot/WEB-INF/classes/atacarnet/src/com/evi/modules/atacarnet/action/

  2.DebitNoteAction.class.getResource("/")

  得到的是当前的classpath的绝对URI路径。

  如:file:/D:/eclipse/springTest/WebRoot/WEB-INF/classes/

  3.Thread.currentThread().getContextClassLoader().getResource("")

  得到的也是当前ClassPath的绝对URI路径

  如:file:/D:/eclipse/springTest/WebRoot/WEB-INF/classes/

  4.DebitNoteAction.class.getClassLoader().getResource("") 或ClassLoader.getSystemResource("")

  得到的也是当前ClassPath的绝对URI路径。

  如:file:/D:/eclipse/springTest/WebRoot/WEB-INF/classes/

  5.取得服务器相对路径

  System.getProperty("user.dir")

  例如:E:\apache-tomcat-5.5.16\apache-tomcat-5.5.16\bin

  我推荐使用Thread.currentThread().getContextClassLoader().getResource("")来得到当前的classpath的绝对路径的URI表示法

  6.取得项目中的绝对路径

  一般用request.getRealPath("/")或request.getRealPath("/config/")

  但现在不提倡使用request.getRealPath("/")了,大家可试用ServletContext.getRealPath("/")方法得到Web应用程序的根目录的绝对路径

  要取得src的文件非常容易,因为src是默认的相对目录,比如你说要取得src下com目录的test.java文件,你只需要这样就够了

  File f = new File(com/test.java);

  但如果我要取得不在src目录或者WebRoot目录下的文件呢,而是要从src或者WebRoot同级的目录中取呢,比如说doc吧

  我的硬方法是这样实现的:

  String path = this.getServletContext().getRealPath("/");

  Properties p = new Properties();

  p.load(new FileInputStream(new File(path.substring(0,(path.lastIndexOf("\\WebRoot") + 1)) + "doc/db.properties")));

  System.out.println(p.getProperty("driverName"));

posted @ 2013-10-18 21:29 思无 阅读(11263) | 评论 (1)编辑 收藏

2012年4月18日 #

Description

这个包增强了标准jdk的java.lang包,重用性很高。

顶层的包里面包含了一些工具类,同时一些子包里面,包含了math,concurrent和builder。使用这些工具类很方便。子包中可能包含一些需要实现的借口和类。他们更像了全局的功能实现。

Lang 3.0 is JDK 1.5+; before that Lang was JDK 1.2+. In both cases you can find features of later JDKs being maintained by us and likely to be removed or modified in favour of the JDK in the next major version. Note that Lang 3.0 uses a different package than its predecessors, allowing it to be used at the same time as an earlier version.

Lang 3.0需要jdk 1.5+,之前的版本只需要1.2+。

You will find deprecated methods as you stroll through the Lang documentation. These are removed in the next major version.

Before we begin, it's a good time to mention the Utils classes. They all contain empty public constructors with warnings not to use. This may seem an odd thing to do, but it allows tools like Velocity to access the class as if it were a bean. In other words, yes we know about private constructors and have chosen not to use them.

lang3.*

String manipulation - StringUtils, StringEscapeUtils, RandomStringUtils, Tokenizer, WordUtils

Lang has a series of String utilities. The first is StringUtils, oodles and oodles of functions which tweak, transform, squeeze and cuddle java.lang.Strings. In addition to StringUtils, there are a series of other String manipulating classes; RandomStringUtils, StringEscapeUtils and Tokenizer. RandomStringUtils speaks for itself. It's provides ways in which to generate pieces of text, such as might be used for default passwords. StringEscapeUtils contains methods to escape and unescape Java, JavaScript, HTML, XML and SQL. Tokenizer is an improved alternative to java.util.StringTokenizer.

These are ideal classes to start using if you're looking to get into Lang. StringUtils' capitalize, substringBetween/Before/After, split and join are good methods to begin with. If you use java.sql.Statements a lot, StringEscapeUtils.escapeSql might be of interest.

In addition to these classes, WordUtils is another String manipulator. It works on Strings at the word level, for example WordUtils.capitalize will capitalize every word in a piece of text. WordUtils also contains methods to wrap text.

Character handling - CharSetUtils, CharSet, CharRange, CharUtils

In addition to dealing with Strings, it's also important to deal with chars and Characters. CharUtils exists for this purpose, while CharSetUtils exists for set-manipulation of Strings. Be careful, although CharSetUtils takes an argument of type String, it is only as a set of characters. For example, CharSetUtils.delete("testtest", "tr") will remove all t's and all r's from the String, not just the String "tr".

CharRange and CharSet are both used internally by CharSetUtils, and will probaby rarely be used.

JVM interaction - SystemUtils, CharEncoding

SystemUtils is a simple little class which makes it easy to find out information about which platform you are on. For some, this is a necessary evil. It was never something I expected to use myself until I was trying to ensure that Commons Lang itself compiled under JDK 1.2. Having pushed out a few JDK 1.3 bits that had slipped in (Collections.EMPTY_MAP is a classic offender), I then found that one of the Unit Tests was dying mysteriously under JDK 1.2, but ran fine under JDK 1.3. There was no obvious solution and I needed to move onwards, so the simple solution was to wrap that particular test in a if(SystemUtils.isJavaVersionAtLeast(1.3f)) {, make a note and move on.

The CharEncoding class is also used to interact with the Java environment and may be used to see which character encodings are supported in a particular environment.

Serialization - SerializationUtils, SerializationException

Serialization doesn't have to be that hard! A simple util class can take away the pain, plus it provides a method to clone an object by unserializing and reserializing, an old Java trick.

Assorted functions - ObjectUtils, ClassUtils, ArrayUtils, BooleanUtils

Would you believe it, ObjectUtils contains handy functions for Objects, mainly null-safe implementations of the methods on java.lang.Object.

ClassUtils is largely a set of helper methods for reflection. Of special note are the comparators hidden away in ClassUtils, useful for sorting Class and Package objects by name; however they merely sort alphabetically and don't understand the common habit of sorting java and javax first.

Next up, ArrayUtils. This is a big one with many methods and many overloads of these methods so it is probably worth an in depth look here. Before we begin, assume that every method mentioned is overloaded for all the primitives and for Object. Also, the short-hand 'xxx' implies a generic primitive type, but usually also includes Object.

  • ArrayUtils provides singleton empty arrays for all the basic types. These will largely be of use in the Collections API with its toArray methods, but also will be of use with methods which want to return an empty array on error.
  • add(xxx[], xxx) will add a primitive type to an array, resizing the array as you'd expect. Object is also supported.
  • clone(xxx[]) clones a primitive or Object array.
  • contains(xxx[], xxx) searches for a primitive or Object in a primitive or Object array.
  • getLength(Object) returns the length of any array or an IllegalArgumentException if the parameter is not an array. hashCode(Object), equals(Object, Object), toString(Object)
  • indexOf(xxx[], xxx) and indexOf(xxx[], xxx, int) are copies of the classic String methods, but this time for primitive/Object arrays. In addition, a lastIndexOf set of methods exists.
  • isEmpty(xxx[]) lets you know if an array is zero-sized or null.
  • isSameLength(xxx[], xxx[]) returns true if the arrays are the same length.
  • Along side the add methods, there are also remove methods of two types. The first type remove the value at an index, remove(xxx[], int), while the second type remove the first value from the array, remove(xxx[], xxx).
  • Nearing the end now. The reverse(xxx[]) method turns an array around.
  • The subarray(xxx[], int, int) method splices an array out of a larger array.
  • Primitive to primitive wrapper conversion is handled by the toObject(xxx[]) and toPrimitive(Xxx[]) methods.

Lastly, ArrayUtils.toMap(Object[]) is worthy of special note. It is not a heavily overloaded method for working with arrays, but a simple way to create Maps from literals.

Using toMap
 
       Map colorMap = MapUtils.toMap(new String[][] {{
           {"RED", "#FF0000"},
           {"GREEN", "#00FF00"},
           {"BLUE", "#0000FF"}
       });
     

Our final util class is BooleanUtils. It contains various Boolean acting methods, probably of most interest is the BooleanUtils.toBoolean(String) method which turns various positive/negative Strings into a Boolean object, and not just true/false as with Boolean.valueOf.

Flotsam - BitField, Validate

On reaching the end of our package, we are left with a couple of classes that haven't fit any of the topics so far.

The BitField class provides a wrapper class around the classic bitmask integer, whilst the Validate class may be used for assertions (remember, we support Java 1.2).

lang3.builder.*

When you write a hashcode, do you check Bloch's Effective Java? No? You just hack in a quick number? Well HashCodeBuilder will save your day. It, and its buddies (EqualsBuilder, CompareToBuilder, ToStringBuilder), take care of the nasty bits while you focus on the important bits, like which fields will go into making up the hashcode.

lang3.math.*

Although Commons-Math also exists, some basic mathematical functions are contained within Lang. These include classes to a Fraction class, various utilities for random numbers, and the flagship class, NumberUtils which contains a handful of classic number functions.

There are two aspects of this package I would like to highlight. The first is NumberUtils.createNumber(String), a method which does its best to convert a String into a Number object. You have no idea what type of Number it will return, so you should call the relevant xxxValue method when you reach the point of needing a number. NumberUtils also has a related isNumber method.

lang3.mutable.*

New in 2.1, the mutable package provides mutable wrappers for primitive values (such as int, long, etc.) and Object. These wrappers are simiar to the wrappers provided by the Java API, but allow the wrapped value to be changed without needing to create a separate wrapper object.

lang3.text.*

The text package was added in Lang 2.2. It provides, amongst other classes, a replacement for StringBuffer named StrBuilder, a class for substituting variables within a String named StrSubstitutor and a replacement for StringTokenizer named StrTokenizer. While somewhat ungainly, the Str prefix has been used to ensure we don't clash with any current or future standard Java classes.

lang3.time.*

Lang 2.0 saw the arrival of a time package. It contains some basic utilities for manipulating time (a delorean, police box and grandfather clock?). These include a StopWatch for simple performance measurements and an optimised FastDateFormat class.

New in Lang 2.1 is the DurationFormatUtils class, which provides various methods for formatting durations.

lang3.concurrent.*

In Lang 3.0 a new concurrent package was introduced containing interfaces and classes to support programming with multiple threads. Its aim is to serve as an extension of the java.util.concurrent package of the JDK.

Concurrent initializers

A group of classes deals with the correct creation and initialization of objects that are accessed by multiple threads. All these classes implement the ConcurrentInitializer interface which provides just a single method:

public interface ConcurrentInitializer<T> {
    T get() throws ConcurrentException;
}
   

A ConcurrentInitializer produces an object. By calling the get() method the object managed by the initializer can be obtained. There are different implementations of the interface available addressing various use cases:

ConstantInitializer is a very straightforward implementation of the ConcurrentInitializer interface: An instance is passed an object when it is constructed. In its get() method it simply returns this object. This is useful, for instance in unit tests or in cases when you want to pass a specific object to a component which expects a ConcurrentInitializer.

The LazyInitializer class can be used to defer the creation of an object until it is actually used. This makes sense, for instance, if the creation of the object is expensive and would slow down application startup or if the object is needed only for special executions. LazyInitializer implements the double-check idiom for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd edition, item 71. It uses volatile fields to reduce the amount of synchronization. Note that this idiom is appropriate for instance fields only. For static fields there are superior alternatives.

We provide an example use case to demonstrate the usage of this class: A server application uses multiple worker threads to process client requests. If such a request causes a fatal error, an administrator is to be notified using a special messaging service. We assume that the creation of the messaging service is an expensive operation. So it should only be performed if an error actually occurs. Here is where LazyInitializer comes into play. We create a specialized subclass for creating and initializing an instance of our messaging service. LazyInitializer declares an abstract initialize() method which we have to implement to create the messaging service object:

public class MessagingServiceInitializer
  extends LazyInitializer<MessagingService> {
    protected MessagingService initialize() throws ConcurrentException {
        // Do all necessary steps to create and initialize the service object
        MessagingService service = ...

        return service;
    }
}
   

Now each server thread is passed a reference to a shared instance of our new MessagingServiceInitializer class. The threads run in a loop processing client requests. If an error is detected, the messaging service is obtained from the initializer, and the administrator is notified:

public class ServerThread implements Runnable {
    /** The initializer for obtaining the messaging service. */
    private final ConcurrentInitializer<MessagingService> initializer;

    public ServerThread(ConcurrentInitializer<MessagingService> init) {
        initializer = init;
    }

    public void run() {
        while (true) {
            try {
                // wait for request
                // process request
            } catch (FatalServerException ex) {
                // get messaging service
                try {
                    MessagingService svc = initializer.get();
                    svc.notifyAdministrator(ex);
                } catch (ConcurrentException cex) {
                    cex.printStackTrace();
                }
            }
        }
    }
}
   

The AtomicInitializer class is very similar to LazyInitializer. It serves the same purpose: to defer the creation of an object until it is needed. The internal structure is also very similar. Again there is an abstract initialize() method which has to be implemented by concrete subclasses in order to create and initialize the managed object. Actually, in our example above we can turn the MessagingServiceInitializer into an atomic initializer by simply changing the extends declaration to refer to AtomicInitializer<MessagingService> as super class.

The difference between AtomicInitializer and LazyInitializer is that the former uses classes from the java.util.concurrent.atomic package for its implementation (hence the name). This has the advantage that no synchronization is needed, thus the implementation is usually more efficient than the one of the LazyInitializer class. However, there is one drawback: Under high load, if multiple threads access the initializer concurrently, it is possible that the initialize() method is invoked multiple times. The class guarantees that get() always returns the same object though; so objects created accidently are immideately discarded.

With AtomicSafeInitializer there is yet another variant implementing the lazy initializing pattern. Its implementation is close to AtomicInitializer; it also uses atomic variables internally and therefore does not need synchronization. The name "Safe" is derived from the fact that it implements an additional check which guarantees that the initialize() method is called only once. So it behaves exactly in the same way as LazyInitializer.

Now, which one of the lazy initializer implementations should you use? First of all we have to state that is is problematic to give general recommendations regarding the performance of these classes. The initializers make use of low-level functionality whose efficiency depends on multiple factors including the target platform and the number of concurrent threads. So developers should make their own benchmarks in scenarios close to their specific use cases. The following statements are rules of thumb which have to be verified in practice.

AtomicInitializer is probably the most efficient implementation due to its lack of synchronization and further checks. Its main drawback is that the initialize() method can be called multiple times. In cases where this is not an issue AtomicInitializer is a good choice. AtomicSafeInitializer and LazyInitializer both guarantee that the initialization method is called only once. Because AtomicSafeInitializer does not use synchronization it is probably slightly more efficient than LazyInitializer, but the concrete numbers might depend on the level of concurrency.

Another implementation of the ConcurrentInitializer interface is BackgroundInitializer. It is again an abstract base class with an initialize() method that has to be defined by concrete subclasses. The idea of BackgroundInitializer is that it calls the initialize() method in a separate worker thread. An application creates a background initializer and starts it. Then it can continue with its work while the initializer runs in parallel. When the application needs the results of the initializer it calls its get() method. get() blocks until the initialization is complete. This is useful for instance at application startup. Here initialization steps (e.g. reading configuration files, opening a database connection, etc.) can be run in background threads while the application shows a splash screen and constructs its UI.

As a concrete example consider an application that has to read the content of a URL - maybe a page with news - which is to be displayed to the user after login. Because loading the data over the network can take some time a specialized implementation of BackgroundInitializer can be created for this purpose:

public class URLLoader extends BackgroundInitializer<String> {
    /** The URL to be loaded. */
    private final URL url;

    public URLLoader(URL u) {
        url = u;
    }

    protected String initialize() throws ConcurrentException {
      try {
          InputStream in = url.openStream();
          // read content into string
          ...
          return content;
      } catch (IOException ioex) {
          throw new ConcurrentException(ioex);
      }
    }
}
   

An application creates an instance of URLLoader and starts it. Then it can do other things. When it needs the content of the URL it calls the initializer's get() method:

URL url = new URL("http://www.application-home-page.com/");
URLLoader loader = new URLLoader(url);
loader.start(); // this starts the background initialization

// do other stuff
...
// now obtain the content of the URL
String content;
try {
    content = loader.get(); // this may block
} catch (ConcurrentException cex) {
    content = "Error when loading URL " + url;
}
// display content
   

Related to BackgroundInitializer is the MultiBackgroundInitializer class. As the name implies, this class can handle multiplie initializations in parallel. The basic usage scenario is that a MultiBackgroundInitializer instance is created. Then an arbitrary number of BackgroundInitializer objects is added using the addInitializer() method. When adding an initializer a string has to be provided which is later used to obtain the result for this initializer. When all initializers have been added the start() method is called. This starts processing of all initializers. Later the get() method can be called. It waits until all initializers have finished their initialization. get() returns an object of type MultiBackgroundInitializer.MultiBackgroundInitializerResults. This object provides information about all initializations that have been performed. It can be checked whether a specific initializer was successful or threw an exception. Of course, all initialization results can be queried.

With MultiBackgroundInitializer we can extend our example to perform multiple initialization steps. Suppose that in addition to loading a web site we also want to create a JPA entity manager factory and read a configuration file. We assume that corresponding BackgroundInitializer implementations exist. The following example fragment shows the usage of MultiBackgroundInitializer for this purpose:

MultiBackgroundInitializer initializer = new MultiBackgroundInitializer();
initializer.addInitializer("url", new URLLoader(url));
initializer.addInitializer("jpa", new JPAEMFInitializer());
initializer.addInitializer("config", new ConfigurationInitializer());
initializer.start(); // start background processing

// do other interesting things in parallel
...
// evaluate the results of background initialization
MultiBackgroundInitializer.MultiBackgroundInitializerResults results =
    initializer.get();
String urlContent = (String) results.getResultObject("url");
EntityManagerFactory emf =
    (EntityManagerFactory) results.getResultObject("jpa");
...
   

The child initializers are added to the multi initializer and are assigned a unique name. The object returned by the get() method is then queried for the single results using these unique names.

If background initializers - including MultiBackgroundInitializer - are created using the standard constructor, they create their own ExecutorService which is used behind the scenes to execute the worker tasks. It is also possible to pass in an ExecutorService when the initializer is constructed. That way client code can configure the ExecutorService according to its specific needs; for instance, the number of threads available could be limited.

Utility classes

Another group of classes in the new concurrent package offers some generic functionality related to concurrency. There is the ConcurrentUtils class with a bunch of static utility methods. One focus of this class is dealing with exceptions thrown by JDK classes. Many JDK classes of the executor framework throw exceptions of type ExecutionException if something goes wrong. The root cause of these exceptions can also be a runtime exception or even an error. In typical Java programming you often do not want to deal with runtime exceptions directly; rather you let them fall through the hierarchy of method invocations until they reach a central exception handler. Checked exceptions in contrast are usually handled close to their occurrence. With ExecutionException this principle is violated. Because it is a checked exception, an application is forced to handle it even if the cause is a runtime exception. So you typically have to inspect the cause of the ExecutionException and test whether it is a checked exception which has to be handled. If this is not the case, the causing exception can be rethrown.

The extractCause() method of ConcurrentUtils does this work for you. It is passed an ExecutionException and tests its root cause. If this is an error or a runtime exception, it is directly rethrown. Otherwise, an instance of ConcurrentException is created and initialized with the root cause. (ConcurrentException is a new exception class in the o.a.c.l.concurrent package.) So if you get such a ConcurrentException, you can be sure that the original cause for the ExecutionException was a checked exception. For users who prefer runtime exceptions in general there is also an extractCauseUnchecked() method which behaves like extractCause(), but returns the unchecked exception ConcurrentRuntimeException instead.

In addition to the extractCause() methods there are corresponding handleCause() methods. These methods extract the cause of the passed in ExecutionException and throw the resulting ConcurrentException or ConcurrentRuntimeException. This makes it easy to transform an ExecutionException into a ConcurrentException ignoring unchecked exceptions:

Future<Object> future = ...;
try {
    Object result = future.get();
    ...
} catch (ExecutionException eex) {
    ConcurrentUtils.handleCause(eex);
}
   

There is also some support for the concurrent initializers introduced in the last sub section. The initialize() method is passed a ConcurrentInitializer object and returns the object created by this initializer. It is null-safe. The initializeUnchecked() method works analogously, but a ConcurrentException throws by the initializer is rethrown as a ConcurrentRuntimeException. This is especially useful if the specific ConcurrentInitializer does not throw checked exceptions. Using this method the code for requesting the object of an initializer becomes less verbose. The direct invocation looks as follows:

ConcurrentInitializer<MyClass> initializer = ...;
try {
    MyClass obj = initializer.get();
    // do something with obj
} catch (ConcurrentException cex) {
    // exception handling
}
   

Using the initializeUnchecked() method, this becomes:

ConcurrentInitializer<MyClass> initializer = ...;
MyClass obj = ConcurrentUtils.initializeUnchecked(initializer);
// do something with obj
   

Another utility class deals with the creation of threads. When using the Executor framework new in JDK 1.5 the developer usually does not have to care about creating threads; the executors create the threads they need on demand. However, sometimes it is desired to set some properties of the newly created worker threads. This is possible through the ThreadFactory interface; an implementation of this interface has to be created and pased to an executor on creation time. Currently, the JDK does not provide an implementation of ThreadFactory, so one has to start from scratch.

With BasicThreadFactory Commons Lang has an implementation of ThreadFactory that works out of the box for many common use cases. For instance, it is possible to set a naming pattern for the new threads, set the daemon flag and a priority, or install a handler for uncaught exceptions. Instances of BasicThreadFactory are created and configured using the nested Builder class. The following example shows a typical usage scenario:

BasicThreadFactory factory = new BasicThreadFactory.Builder()
    .namingPattern("worker-thread-%d")
    .daemon(true)
    .uncaughtExceptionHandler(myHandler)
    .build();
ExecutorService exec = Executors.newSingleThreadExecutor(factory);
   

The nested Builder class defines some methods for configuring the new BasicThreadFactory instance. Objects of this class are immutable, so these attributes cannot be changed later. The naming pattern is a string which can be passed to String.format(). The placeholder %d is replaced by an increasing counter value. An instance can wrap another ThreadFactory implementation; this is achieved by calling the builder's wrappedFactory() method. This factory is then used for creating new threads; after that the specific attributes are applied to the new thread. If no wrapped factory is set, the default factory provided by the JDK is used.

Synchronization objects

The concurrent package also provides some support for specific synchronization problems with threads.

TimedSemaphore allows restricted access to a resource in a given time frame. Similar to a semaphore, a number of permits can be acquired. What is new is the fact that the permits available are related to a given time unit. For instance, the timed semaphore can be configured to allow 10 permits in a second. Now multiple threads access the semaphore and call its acquire() method. The semaphore keeps track about the number of granted permits in the current time frame. Only 10 calls are allowd; if there are further callers, they are blocked until the time frame (one second in this example) is over. Then all blocking threads are released, and the counter of available permits is reset to 0. So the game can start anew.

What are use cases for TimedSemaphore? One example is to artificially limit the load produced by multiple threads. Consider a batch application accessing a database to extract statistical data. The application runs multiple threads which issue database queries in parallel and perform some calculation on the results. If the database to be processed is huge and is also used by a production system, multiple factors have to be balanced: On one hand, the time required for the statistical evaluation should not take too long. Therefore you will probably use a larger number of threads because most of its life time a thread will just wait for the database to return query results. On the other hand, the load on the database generated by all these threads should be limited so that the responsiveness of the production system is not affected. With a TimedSemaphore object this can be achieved. The semaphore can be configured to allow e.g. 100 queries per second. After these queries have been sent to the database the threads have to wait until the second is over - then they can query again. By fine-tuning the limit enforced by the semaphore a good balance between performance and database load can be established. It is even possible to change the number of available permits at runtime. So this number can be reduced during the typical working hours and increased at night.

The following code examples demonstrate parts of the implementation of such a scenario. First the batch application has to create an instance of TimedSemaphore and to initialize its properties with default values:

TimedSemaphore semaphore = new TimedSemaphore(1, TimeUnit.SECONDS, 100);
   

Here we specify that the semaphore should allow 100 permits in one second. This is effectively the limit of database queries per second in our example use case. Next the server threads issuing database queries and performing statistical operations can be initialized. They are passed a reference to the semaphore at creation time. Before they execute a query they have to acquire a permit.

public class StatisticsTask implements Runnable {
    /** The semaphore for limiting database load. */
    private final TimedSemaphore semaphore;

    public StatisticsTask(TimedSemaphore sem, Connection con) {
        semaphore = sem;
        ...
    }

    /**
     * The main processing method. Executes queries and evaluates their results.
     */
    public void run() {
        try {
            while (!isDone()) {
                semaphore.acquire();    // enforce the load limit

                executeAndEvaluateQuery();
            }
        } catch (InterruptedException iex) {
            // fall through
        }
    }
}
   

The important line here is the call to semaphore.acquire(). If the number of permits in the current time frame has not yet been reached, the call returns immediately. Otherwise, it blocks until the end of the time frame. The last piece missing is a scheduler service which adapts the number of permits allowed by the semaphore according to the time of day. We assume that this service is pretty simple and knows only two different time slots: working shift and night shift. The service is triggered periodically. It then determines the current time slot and configures the timed semaphore accordingly.

public class SchedulerService {
    /** The semaphore for limiting database load. */
    private final TimedSemaphore semaphore;
    ...

    /**
     * Configures the timed semaphore based on the current time of day. This
     * method is called periodically.
     */
    public void configureTimedSemaphore() {
        int limit;
        if (isWorkshift()) {
            limit = 50;    // low database load
        } else {
            limit = 250; // high database load
        }

        semaphore.setLimit(limit);
    }
}
   

With the setLimit() method the number of permits allowed for a time frame can be changed. There are some other methods for querying the internal state of a timed semaphore. Also some statistical data is available, e.g. the average number of acquire() calls per time frame. When a timed semaphore is no more needed, its shutdown() method has to be called.

posted @ 2012-04-18 13:47 思无 阅读(1059) | 评论 (0)编辑 收藏

2012年4月16日 #

Maven的功能
  • 遵循最佳实践,简单的建立项目-在分秒之间建立一个新的项目或者模块。
  • 在多个项目之间统一使用。
  • 依附环境的管理,包括自动的升级。
  • 方便地在同一时间参加多个项目。
  • 大且不断增长的库和元数据可供食用。
  • 可扩展,用java或脚本语言开发插件
  • 加入新的功能只需要很少或不需要额外的配置
  • Ant任务支持依附环境管理,在Maven之外进行发布。
  • 在大多数情况下,maven不需要脚本就可以编译多个项目,打包成jar, war或基于元数据进行项目发布
  • 清晰的项目信息:Maven能够建立一个站点或PDF文档,其中包括项目的一些基本信息和报表。
  • 版本管理和项目发布:不需要太多的额外配置,maven会和你的代码管理器整合,然后基于一定的tag,来帮助管理你的版本。它也可以为其他的项目发布到一个地方。maven也支持发布成jar或其他归档的文件或是源代码。
  • 依附管理:maven建议集中管理jar库。项目客户可以到集中的jar库中下载任何包。这允许maven用户可以在多个项目中公用jar,并鼓励项目之间的交流。

 

Maven如何帮助改进我们的开发过程?

通过标准的约定和实践Maven对你的build流程提供帮助,加速你的开发周期并同时帮助你的项目成功。

下面我们提供一些例子让你来运行maven!

如何建立Maven?

基本上,maven默认的配置已经足够面对大部分情况了,但是如果你需要修改缓存策略或者设置http代理,你就需要自己建立一个配置。详细请参考Guide to Configuring Maven

 

如何建立我的第一个maven项目?

我们会使用maven的archetype 机制来建立我们的第一个maven项目。maven中,archetype是项目模版,在加上一些用户的输入来建立一个maven的项目。我们会向你展示archetype机制如何工作。更多请参考

 


创建最简单的maven项目,只需要执行下面的命令就可以了

mvn archetype:generate \   -DarchetypeGroupId=org.apache.maven.archetypes \   -DgroupId=com.mycompany.app \   -DartifactId=my-app 

执行命令后,首先,你会发现一个叫my-app的文件夹建立了,其中有pom.xml文件,内容如下:

<project xmlns="http://maven.apache.org/POM/4.0.0"   
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<packaging>jar</packaging>
<version>1.0-SNAPSHOT</version>
<name>Maven Quick Start Archetype</name>
<url>http://maven.apache.org</url>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>

pom.xml里有项目的project object model。pom是maven的基本工作单元。This is important to remember because Maven is inherently project-centric in that everything revolves around the notion of a project. 简单的说,pom包含了所有项目的重要信息。更多请参考Introduction to the POM.

This is a very simple POM but still displays the key elements every POM contains, so let's walk through each of them


to familiarize you with the POM essentials:

  • project This is the top-level element in all Maven pom.xml files.
  • modelVersion This element indicates what version of the object model this POM is using. The version of the model itself changes very infrequently but it is mandatory in order to ensure stability of use if and when the Maven developers deem it necessary to change the model.
  • groupId This element indicates the unique identifier of the organization or group that created the project. The groupId is one of the key identifiers of a project and is typically based on the fully qualified domain name of your organization. For example org.apache.maven.plugins is the designated groupId for all Maven plug-ins.
  • artifactId This element indicates the unique base name of the primary artifact being generated by this project. The primary artifact for a project is typically a JAR file. Secondary artifacts like source bundles also use the artifactId as part of their final name. A typical artifact produced by Maven would have the form <artifactId>-<version>.<extension> (for example, myapp-1.0.jar).
  • packaging This element indicates the package type to be used by this artifact (e.g. JAR, WAR, EAR, etc.). This not only means if the artifact produced is JAR, WAR, or EAR but can also indicate a specific lifecycle to use as part of the build process. (The lifecycle is a topic we will deal with further on in the guide. For now, just keep in mind that the indicated packaging of a project can play a part in customizing the build lifecycle.) The default value for the packaging element is JAR so you do not have to specify this for most projects.
  • version This element indicates the version of the artifact generated by the project. Maven goes a long way to help you with version management and you will often see the SNAPSHOT designator in a version, which indicates that a project is in a state of development. We will discuss the use of snapshots and how they work further on in this guide.
  • name This element indicates the display name used for the project. This is often used in Maven's generated documentation.
  • url This element indicates where the project's site can be found. This is often used in Maven's generated documentation.
  • description This element provides a basic description of your project. This is often used in Maven's generated documentation.

For a complete reference of what elements are available for use in the POM please refer to our POM Reference. Now let's get back to the project at hand.

After the archetype generation of your first project you will also notice that the following directory structure has been created:

my-app |-- pom.xml `-- src     |-- main     |   `-- java     |       `-- com     |           `-- mycompany     |               `-- app     |                   `-- App.java     `-- test         `-- java             `-- com                 `-- mycompany                     `-- app                         `-- AppTest.java 

As you can see, the project created from the archetype has a POM, a source tree for your application's sources and a source tree for your test sources. This is the standard layout for Maven projects (the application sources reside in ${basedir}/src/main/java and test sources reside in ${basedir}/src/test/java, where ${basedir} represents the directory containing pom.xml).

If you were to create a Maven project by hand this is the directory structure that we recommend using. This is a Maven convention and to learn more about it you can read our Introduction to the Standard Directory Layout.

Now that we have a POM, some application sources, and some test sources you are probably asking ...


 









 

posted @ 2012-04-16 17:05 思无 阅读(354) | 评论 (0)编辑 收藏

2012年4月6日 #

     摘要: JQGrid是一个在jquery基础上做的一个表格控件,以ajax的方式和服务器端通信。JQGrid Demo 是一个在线的演示项目。在这里,可以知道jqgrid可以做什么事情。下面是转自其他人blog的一个学习资料,与其说是学习资料,说成查询帮助文档更加合适。 jqGrid学习之 ------------- 安装 jqGrid安装很简单,只需把相应的css、js文件加入到页面中即可。 按照...  阅读全文
posted @ 2012-04-06 12:57 思无 阅读(56568) | 评论 (4)编辑 收藏

2012年2月20日 #

1. http://download.eclipse.org/windowbuilder/WB/integration/3.7/
1.http://code.google.com/p/visualswing4eclipse/
2个用于J2SE swing之类的桌面编程。

2. doxygen    http://www.stack.nl/~dimitri/doxygen/
能根据项目的源文件,自动生成比较完整的文档,包括类层次图
3. open explorer   http://blog.samsonis.me/2009/02/open-explorer-plugin-for-eclipse/
可以在eclipse中比较方便的打开本地文件夹
posted @ 2012-02-20 14:25 思无 阅读(258) | 评论 (0)编辑 收藏

2011年8月23日 #

svn作为一个代码版本管理工具,适合团队使用。
1. 服务端架设
平台选windows. 软件下载:http://subversion.tigris.org/servlets/ProjectDocumentList?expandFolder=91&folderID=91
相关命令
sc create svn binpath= "\"E:\SVN\svn-win32-1.4.3\bin\svnserve.exe\" --service -r E:\SVN\MyRePos" displayname= "Subversion Server" depend= Tcpip start= auto 建立svn服务器
net stop svn 停止svn服务
net start svn 启动svn服务
sc delete svn 删除svn服务,可能需要重启
SVNADMIN CREATE E:\SVN\MyRePos 建立版本库
svn import -m "MyProject" LoginDemo file:///E:\SVN\MyRePos\LoginDemo 导入项目LoginDemo在当前目录下

2.客户端插件

eclipse安装插件,在eclipse上打开资源库,就可以将资源库中的工程检出了。

因为只有在自己的机器上进行操作,所以没有涉及到用户权限设置之类的问题.

Issue:
使用svn:协议连接server出现无法连接,首先需要查看服务有没有启动。
遇到错误消息‘object expected’,可能是配置文件的键值对有空格。



posted @ 2011-08-23 00:15 思无 阅读(221) | 评论 (0)编辑 收藏

2011年8月7日 #

项目通过JASPER跑报表,有几个报表牵涉的数据量比较大,导致响应时间超过1个小时,此时页面会提示“页面无法显示”。
开始以为是session的超时,于是在页面中添加了ajax去定期发送请求给服务器的代码来维持session, 问题无法解决。
然后以为是页面缓存的问题,在请求中加了随机的数字,查看每次请求回来的响应头状态都是200的,问题仍旧无法解决。
继续百度,继续谷歌。
搜索到microsoft的一篇文章,如下
http://support.microsoft.com/kb/181050 

Internet Explorer error "connection timed out" when server does not respond
问题得到解决。
其实可以说问题无法得到解决。我怎么可能让每个用户去修改他们的注册表么。

看来只能采取异步的请求方式了。

posted @ 2011-08-07 22:12 思无 阅读(461) | 评论 (0)编辑 收藏

2011年7月20日 #

JsonConfig config = new JsonConfig();
  config.setJsonPropertyFilter(new PropertyFilter() {
   public boolean apply(Object source, String name, Object value) {
    if (name.equals("form") || name.equals("formElm")   ) {
     return true;
    } else {
     return false;
    }
   }
  });
  
  JSONObject jsonObject = JSONObject.fromObject( form, config );
posted @ 2011-07-20 16:14 思无 阅读(2551) | 评论 (0)编辑 收藏

2011年6月29日 #


下面的方法提供了比较好的数据库分割字符串的方法。不要在每次需要这种功能的地方各写一个,统一的工具类非常重要。更能排除以下低级的错误。
create
 or replace
PACKAGE pk_genutilitypkg
AS
   
/*

   Generic String Parser: provide a delimiter and it returns an
   index-by table of the individual elements of the string that are
   separated by the specified delimiter.

   Author: "GAUTHAM CHANDRASEKARAN" <gautha@hotmail.com>
   
*/
   TYPE t_string 
IS TABLE OF VARCHAR2(2000)
      
INDEX BY BINARY_INTEGER;

   m_ctr 
NUMBER(5);
   m_pos 
NUMBER(5);

   
PROCEDURE sp_parsestring (
      p_string 
IN VARCHAR2,
      delimiter 
IN VARCHAR2,
      p_t_string OUT t_string);
END pk_genutilitypkg;
  
create or replace
PACKAGE BODY pk_genutilitypkg
AS

   
PROCEDURE sp_parsestring (
      p_string 
IN VARCHAR2,
      delimiter 
IN VARCHAR2,
      p_t_string OUT t_string)
   
IS
      m_string 
VARCHAR2(4000);
   
BEGIN
      
/* Raise a Error if the length of the delimiter is not 1 */
      
IF LENGTH (delimiter) != 1
      
THEN
         raise_application_error (
-20001,
             
'Delimiter should be of only one character');
         
RETURN;
      
END IF;
      m_string :
= p_string;
      m_ctr :
= 1;
      LOOP
         m_pos :
= INSTR (m_string, delimiter);
         
IF m_pos > 1
         
THEN
            p_t_string (m_ctr) :
= LTRIM(RTRIM( SUBSTR (m_string, 1, m_pos - 1) ));
            
IF (m_pos < LENGTH (m_string))
            
THEN
               m_string :
= SUBSTR (
                              m_string,
                              m_pos 
+ 1,
                              LENGTH (m_string) 
- m_pos
                           );
            ELSIF m_pos 
= LENGTH (m_string)
            
THEN
               m_ctr :
= m_ctr + 1;
               p_t_string (m_ctr) :
= NULL;
               
EXIT;
            
END IF;
         ELSIF m_pos 
= 1
         
THEN
            p_t_string (m_ctr) :
= NULL;
            
IF m_pos < LENGTH (m_string)
            
THEN
               m_string :
= SUBSTR (
                              m_string,
                              m_pos 
+ 1,
                              LENGTH (m_string) 
- m_pos
                           );
            ELSIF m_pos 
= LENGTH (m_string)
            
THEN
               m_ctr :
= m_ctr + 1;
               p_t_string (m_ctr) :
= NULL;
               
EXIT;
            
END IF;
         ELSIF m_pos 
= 0
         
THEN

            p_t_string (m_ctr) :
= LTRIM(RTRIM( m_string ));
            
EXIT;
         
END IF;
         m_ctr :
= m_ctr + 1;
      
END LOOP;
   
END;
END pk_genutilitypkg;

使用方法如下:
PROCEDURE UseStringParse(
    user_list   
IN       VARCHAR2
)
IS
    userid_list   
VARCHAR2 (1000) := user_list;
    userlistarray   pk_genutilitypkg.t_string;
    vuserid       
VARCHAR2 (100);
BEGIN

    pk_genutilitypkg.sp_parsestring(userid_list, 
',', userlistarray);
    
FOR userindex IN 1 .. userlistarray.COUNT
        loop
        vuserid:
=userlistarray(userindex);
    
END LOOP;

END UseStringParse;


posted @ 2011-06-29 14:43 思无 阅读(949) | 评论 (0)编辑 收藏

Actuate 简单报表的建立过程

1. 1. 打开 e.report 选择new report


2. 2.选择空白报表


3. 3. 建立数据库连接。Toolsà Database Connection


4. 4. 填写参数


5. 5. 双击datasource , 填写查询参数


6. 6. 选中Content, Tool--〉 Grouping, 设置整个报表的group1or n层。


7. 7. 单击grouping option,设置group的显示,是否需要befer,after等,每个group需要单独设置。


8.  8. 设置各种页面的控件


9. 9. 设置参数,这些参数可以用在报表控制上或是查询的参数Tools -- Parameters


修改后的查询字段

select fund_id,fund_name,fund_abbreviation,fund_mode,rule_id from te_fund where fund_mode=:fundmode

参数10. 用来控制页面显示

label为例子,重写OnRow方法

   

Sub OnRow( row As AcDataRow )
    Super::OnRow( row )
    
if fundmode="M" then
         me.Text 
= "Manual Fund Name"
    
else
         me.Text 
= "Auto Fund Name"
    
end if
End Sub

11. datasource中的数据控制页面

Sub OnRow( row As AcDataRow )
 Super::OnRow( row )
    
If row.GetValue("DR_SHORT_FLAG"=0 Then
     me.Text 
= "BUY BACK"
    
ElseIf row.GetValue("DR_SHORT_FLAG"=1 Then
     me.Text 
= "Sell to Open"
    
End if
End Sub

 

posted @ 2011-06-29 13:28 思无 阅读(322) | 评论 (0)编辑 收藏