Shulou(Shulou.com)06/02 Report--

This article will explain in detail what are the headaches related to Java 8, and the content of the article is of high quality, so the editor will share it with you for reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

1. Parallel Streams may actually degrade your performance

Java8 brings one of the most anticipated new features-parallelism. The parallelStream () method implements parallelism on collections and streams. It breaks them down into sub-problems and then assigns them to different threads for processing, which can be divided into different CPU cores for processing and then merged together after completion. The implementation principle is mainly based on the fork/join framework. Okay, that sounds cool, right? That must be able to speed up the operation of big data collection in a multi-core environment, right?

No, if you use it incorrectly, it will actually make your code run slower. We did some benchmark tests and found that it was 15% slower, or even worse. Suppose we have run multiple threads and then use .parallelStream () to add more threads to the thread pool, which can easily exceed the upper limit of multicore CPU processing, increasing the number of context switches and slowing down the whole.

The benchmark divides a collection into different groups (primary / non-primary):

Map groupByPrimary = numbers .parallelStream () .collect (Collectors.groupingBy (s-> Utility.isPrime (s)

Performance degradation may also be due to other reasons. If we divide it into multiple tasks, one of the tasks may take much longer than the others for some reason. .parallelStream () breaks down the task, which may be slower than it is as a complete task. Take a look at this article, some examples and code given by Lukas Krecan.

Reminder: parallelism brings many benefits, but there are also other issues to consider. When you are already running in a multithreaded environment, keep this in mind and be familiar with the running mechanism behind it.

2. Shortcomings of Lambda expression

Lambda expression. Oh, lambda expression. We can do almost anything without lambda expressions, but lambda is so elegant and gets rid of annoying code that it's easy to fall in love with lambda. For example, when I wake up in the morning, I want to go through the list of players in the World Cup and know the exact number of players (interesting fact: 254).

List lengths = new ArrayList (); for (String countries: Arrays.asList (args)) {lengths.add (check (country));}

Now let's use a nice lambda expression to do the same thing:

Stream lengths = countries.stream () .map (countries-< check (country))

Wow! This is amazing. Adding new elements like lambda expressions to Java, although it may seem like a good thing, is actually a departure from Java's original specification. Bytecode is completely object-oriented, and with the addition of lambda, there is a great difference in the structure between the actual code and the bytecode at run time. To read more about the negative effects of lambda expressions, see the Tal Weiss article.

At a deeper level, what code you write and what code you debug are two different things. The stack trace is getting larger and larger, making it difficult to debug the code. Some very simple things, such as adding an empty string to list, is supposed to be such a short stack with

At LmbdaMain.check (LmbdaMain.java:19) at LmbdaMain.main (LmbdaMain.java:34)

Like this:

At LmbdaMain.check (LmbdaMain.java:19) at LmbdaMain.lambda$0 (LmbdaMain.java:37) at LmbdaMain$$Lambda$1/821270929.apply (Unknown Source) at java.util.stream.ReferencePipeline$3 $1.accept (ReferencePipeline.java:193) at java.util.Spliterators$ArraySpliterator.forEachRemaining (Spliterators.java:948) at java.util.stream.AbstractPipeline.copyInto (AbstractPipeline.java:512) at java.util.stream.AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:502) at java.util.stream.ReduceOps$ReduceOp.evaluateSequential (ReduceOps.java:708) at java.util. Stream.AbstractPipeline.evaluate (AbstractPipeline.java:234) at java.util.stream.LongPipeline.reduce (LongPipeline.java:438) at java.util.stream.LongPipeline.sum (LongPipeline.java:396) at java.util.stream.ReferencePipeline.count (ReferencePipeline.java:526) at LmbdaMain.main (LmbdaMain.java:39)

Another problem with lambda expressions is about overloading: when they are used to call a method, there are some parameters that can be of multiple types, which can lead to ambiguous calls in some cases. Lukas Eder is illustrated with sample code.

Reminder: to realize this, tracing can be painful at times, but it's not enough to keep us away from valuable lambda expressions.

3. The Default method is distracting.

The Default method allows a default implementation in a functional interface, which is undoubtedly the coolest of the new features of Java8, but it conflicts with the way we used it before. So in that case, why introduce the default method? What if it is not introduced?

The main motivation behind the Defalut method is that if we want to add a method to an existing interface, we can do this without rewriting the implementation and make it compatible with the old version. For example, take this code from the Oracle Java tutorial that adds the ability to specify a time zone:

Public interface TimeClient {/ /... Static public ZoneId getZoneId (String zoneString) {try {return ZoneId.of (zoneString);} catch (DateTimeException e) {System.err.println ("Invalid time zone:" + zoneString + "; using default time zone instead."); return ZoneId.systemDefault ();} default public ZonedDateTime getZonedDateTime (String zoneString) {return ZonedDateTime.of (getLocalDateTime (), getZoneId (zoneString));}}

In this way, the problem was easily solved. Is that right? The Default method mixes the interface and implementation separation. It seems that we no longer have to worry about their own hierarchical structure, and now we need to solve new problems. To learn more, read Oleg Shelajev's article on RebelLabs.

Warning: when you have a hammer in your hand, everything looks like a nail. Remembering their original usage, it makes no sense to keep the original interface while refactoring to introduce new abstract classes.

4. How can I save you, Jagsaw?

The goal of the Jigsaw project is to modularize Java and split JRE into components that can operate with each other. The main motivation behind this is the desire for a better, faster and more powerful Java embedded system. I tried to avoid mentioning the "Internet of things", but I said it anyway. Reducing the size of JAR, improving performance, enhancing security, and so on are what this ambitious project promises.

But where is it? Mark Reinhold, Oracle's * * Java architect, said: Jigsaw, which passed the exploration phase and recently entered the second phase, is now starting to design and implement the product. The project was originally scheduled to be completed in Java8. It is now postponed to Java9, which is likely to become its most important new feature.

Reminder: if that's what you're waiting for, Java9 should be released in 2016. At the same time, if you want to follow closely or even participate, you can add to this mailing list.

5. The problems that still exist

Abnormal detected

No one likes tedious code, which is why lambdas expressions are so popular. Think about annoying exceptions, whether or not you need to logically catch or handle checked exceptions, you need to catch them. Even if some will never happen, an exception like the following will never happen:

Try {httpConn.setRequestMethod ("GET");}   catch (ProtocolException pe) {/ * Why don't you call me anymore? * /}

Original type

They are still there, and it is painful to use them correctly. Primitive types prevent Java from becoming a pure object-oriented language, and removing them has no significant impact on performance. By the way, none of the new JVM languages include primitive types.

Operator overloading

The father of James Gosling,Java, once said in an interview: "I abandoned operator overloading because of my personal subjective reasons, because I have seen too many people abuse it on C++." It makes sense, but many people hold different views. Other JVM languages also provide this functionality, but on the other hand, it causes some code to look like this:

JavascriptEntryPoints

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