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This article mainly shows you "what is the internal construction of JDK in java", which is easy to understand and clear, hoping to help you solve your doubts, the following let the editor lead you to study and learn "what is the internal construction of JDK in java" this article.

0 、 javap

You can pass these useful parameters to javap (Java Class file decompiler):

-I-print lines and local variables

-p-print all class and member information, including non-public

-c-print method bytecode

For example, in the famous "do you really know Classloader?" In the presentation, when a NoSuchMethodException error occurs, we can execute the following command to investigate which member methods are in the class and get all the information that the class is looking for:

Javap-l-c-p Util2

Javap is useful when debugging class internal information or studying random bytecode order.

1 、 jjs

The jjs command launches a JavaScript command terminal that you can use as a calculator or test the odd usage of JS with random JS strings. Don't let another JavaScript puzzle surprise you!

Ha, did you see what just happened? But JavaScript is another topic, and you just need to know that you can use jjs to know how JS works even without node.js or a browser.

2 、 jhat

The Java heap analysis tool (jhat) is exactly what its name describes: analyzing dump heap information. In the following small example, we construct an OutOfMemoryError and then specify-XX:+HeapDumpOnOutOfMemoryError to the java process so that the runtime produces a dump file for us to analyze.

Public class OhMyMemory {private static Map map = new HashMap (); public static void main (String [] args) {Runtime.getRuntime () .addShutdownHook (new Thread () {@ Override public void run () {System.out.println ("We have accumulated" + map.size () + "entries");}}); for (int I = 0; Map.put +) {map.put (Integer.toBinaryString (I), I);}

Generating an OutOfMemoryError is simple (most of the time we don't intend to do it), we just have to keep making garbage collectors that don't work.

Running this code produces the following output:

Org.shelajev.throwaway.jdktools.OhMyMemory java.lang.OutOfMemoryError: Java heap space Dumping heap to java_pid5644.hprof... Heap dump file created [73169721 bytes in 0.645 secs] Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at java.util.HashMap.resize (HashMap.java:703) at java.util.HashMap.putVal (HashMap.java:662) at java.util.HashMap.put (HashMap.java:611) at org.shelajev.throwaway.jdktools.OhMyMemory.main (OhMyMemory.java:24) at sun.reflect.NativeMethodAccessorImpl.invoke0 (NativeMethod) at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62) at Sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke (Method.java:483) at com.intellij.rt.execution.application.AppMain.main (AppMain.java:134) We have accumulated 393217 entries

Yes, we now have a document for analysis. We execute jhat on the file to start the analysis, and jhat will analyze the file and open a http server for us to view the results.

$jhat java_pid5644.hprof Reading from java_pid5644.hprof... Dump file created Thu Aug 14 14:48:19 EEST 2014 Snapshot read, resolving... Resolving 1581103 objects... Chasing references, expect 316 dots... Eliminating duplicate references. Snapshot resolved. Started HTTP server on port 7000 Server is ready.

You can view the data of dump by accessing http://localhost:7000.

On that page we can see what is running out of memory through the bar chart of the heap information.

Now we can clearly see that HashMap with 393567 nodes is the culprit that caused the program to crash. Although there are more tools to check memory distribution usage and heap analysis, jhat is built-in and is a good start for analysis.

3 、 jmap

Jmap is a memory mapping tool that provides another way to get heap dump files without raising an OutOfMemoryErrors. Let's modify the above program a little bit to see the effect.

Public class OhMyMemory {private static Map map = new HashMap (); public static void main (String [] args) {Runtime.getRuntime () .addShutdownHook (new Thread () {@ Override public void run () {try {System.out.println ("Enter something, so I'll release the process"); System.in.read () System.out.println ("We have accumulated" + map.size () + "entries");} catch (IOException e) {e.printStackTrace ();}); for (int I = 0; I < 10000; iTunes +) {map.put (Integer.toBinaryString (I), I);}

Note that now we don't consume a lot of memory, just finish early and wait in the process shutdown hook to keep JVM from exiting. This allows us to use jmap to connect to this process to get the precious memory dump.

So you can use the two functions of jmap to get heap statistics and trigger a heap dump. Therefore, when performing:

Jmap-heap 1354 (where 1354 is the process number of the above program), you can get a good memory usage statistics:

$jmap-heap 1354 Attaching to process ID 1354, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.0-b70 using thread-local object allocation. Parallel GC with 4 thread (s) Heap Configuration: MinHeapFreeRatio = 40 MaxHeapFreeRatio = 70 MaxHeapSize = 67108864 (64.0MB) NewSize = 1572864 (1.5MB) MaxNewSize = 22020096 (21.0MB) OldSize = 45088768 (43.0MB) NewRatio = 2 SurvivorRatio = 8 MetaspaceSize = 21807104 (20.796875MB) CompressedClassSpaceSize = 1073741824 (1024.0MB) MaxMetaspaceSize = 17592186044415 MB G1HeapRegionSize = 0 (0.0MB) Heap Usage: PS Young Generation Eden Space: capacity = 1048576 (1.0MB) used = 628184 (0.5990829467773438MB) free = 420392 (0.40091705322265625MB) 59.908294677734375% used From Space: capacity = 524288 (0.5MB) used = 491568 (0. 4687957763671875MB) free = 32720 (0.0312042236328125MB) 93.75915527343775 used To Space: capacity = 524288 (0.5MB) used = 0 (0.0MB) free = 524288 (0.5MB) 0.0% used PS Old Generation capacity = 45088768 (43.0MB) used = 884736 (0.84375MB) free = 44204032 (42.15625MB) 1.9622093023255813% used 981 interned Strings occupying 64824 bytes. $jmap-dump:live,format=b,file=heap.bin 1354 Dumping heap to / Users/shelajev/workspace_idea/throwaway/heap.bin. Heap dump file created

Jmap can also simply trigger the current heap dump, which can be analyzed at will. You can pass a-dump parameter to jmap as in the following example.

Now that you have the file heap.bin obtained by dump, you can use your favorite memory analysis tool to analyze it.

4 、 jps

Jps is the most commonly used tool for displaying Java program system processes (PID). It has nothing to do with the platform and is very useful. Imagine that we start the above program and then want to connect to it with jmap. At this time, we need the PID,jps of the program to come in handy.

Jps-mlv 5911 com.intellij.rt.execution.application.AppMain org.shelajev.throwaway.jdktools.OhMyMemory-Xmx64m-Didea.launcher.port=7535-Didea.launcher.bin.path=/Applications/IntelliJ IDEA 14 EAP.app/Contents/bin-Dfile.encoding=UTF-8 5544-Dfile.encoding=UTF-8-ea-Dsun.io.useCanonCaches=false-Djava.net.preferIPv4Stack=true-Djsse.enableSNIExtension=false-XX:+UseConcMarkSweepGC-XX:SoftRefLRUPolicyMSPerMB=50-XX:+HeapDumpOnOutOfMemoryError-Xverify:none-Xbootclasspath/a:../lib/boot.jar-Xms128m-Xmx750m- XX:MaxPermSize=350m-XX:ReservedCodeCacheSize=225m-XX:+UseCompressedOops-agentlib:yjpagent=probe_disable=* Disablealloc,disabletracing,onlylocal,disableexceptiontelemetry,delay=10000,sessionname=IntelliJIdea14-Didea.java.redist=NoJavaDistribution-Didea.home.path=/Applications/IntelliJ IDEA 14 EAP.app/Contents-Didea.paths.selector=IntelliJIdea14 5930 sun.tools.jps.Jps-mlvV-Dapplication.home=/Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home-Xms8m

We found that in most cases, the "- mlv" parameter is used in combination. It prints the parameters of the main method, the full package name, and the parameters related to JVM. So you can find the one you want in a lot of similar processes.

Now that you have the file heap.bin obtained by dump, you can use your favorite memory analysis tool to analyze it.

5 、 jstack

Jstack is a thread stack tool that generates a specified JVM process. When your program is circling around all the time, and you want to find out what the thread has done to cause the deadlock, then jstack works best.

Jstack has only a few parameter options, and if you're not sure, add them all. If you find that some information doesn't make much sense to you later, you can adjust the parameters to limit its output.

The-F option can be used to force dump, which is useful when a process is suspended, and the-I option prints synchronization and lock information.

$jstack-F-l 9153 Attaching to process ID 9153, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.0-b70 Deadlock Detection: No deadlocks found. ... .

The output above looks simple, but it contains information about the state of each thread and its current stack.

Jstack is very useful and we use it very frequently in our daily work, especially when we are responsible for starting and stopping the test engine of the application server. Testing often doesn't go well, and jstack lets us know what's going on inside JVM without any negative impact.

-Neeme Praks (ZeroTurnaround Senior Product engineer)

Is there any others?

Today we introduce the great tools pre-installed with the JDK release. Trust me, you will use some of them someday. So, if you have time, you can look through their official documents.

Try using them in different situations and falling in love with them.

If you want to learn some great tools that don't come with JDK, check out JRebel, which allows you to see code changes right away, as well as our new product, XRebel, which scans your web app like X-ray glasses.

If you know how to develop gadgets that are critical in practice, comment at the end of this article or share the details of the tool on @ shelajev on twitter.

Bonus Section: References

Reward link: reference

The following is a more complete list of JDK tools available. Although this is not a complete list, in order to save space, we omitted encryption, web-services-related tools, and so on. Thank you manpagez.com for the resources.

Jar-A tool for creating and managing jar files.

Java-Java application initiator. In this article, this initiator is used for development and deployment.

Javac-Java compiler.

Javadoc-API document generator.

The javah-native native method is used to generate C language header files and source files.

Javap-class file decompiler.

Jcmd-JVM command line diagnostic tool that sends diagnostic command requests to JVM.

Jconsole-A JMX-compatible graphical tool for monitoring JVM. You can monitor local and remote JVM, or you can monitor and manage a single application.

Jdb-Java debugger.

The jps-JVM process viewer lists all hotspot JVM processes that the system is running.

Jstat-JVM status monitoring tool. It can collect and print the specified JVM process performance status.

Jhat-A browser for heap dump information, start a web server to display the heap dump information you get with things like jmap-dump.

Jmap-Java memory mapping tool that prints the specified process, core file, remote debug server shared memory mapping, or heap memory details.

Jsadebugd-Java service debug daemon-attaches to a Java process or core file and acts as a debugging server.

Jstack-Java stack information tool-prints the thread stack of a specified process or core file or remote debugging server.

Jjs-run the Nashorn command line script shell.

Jrunscript-Java script running tool. But you have to know that this is actually a test feature that hasn't been supported yet. It may be removed in future versions of JDK.

These are all the contents of the article "what are the JDK construction projects in java?" Thank you for your reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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