Not envy mandarin duck not envy fairy, a line of code half a day. Original: Taste of Little Sister (wechat official ID: XjjDog), welcome to share, please reserve the source.

A senior developer, using SpringBoot for the first time, decided to print a classic HelloWorld.

He was so excited that he mistyped a letter and became HalloWorld.

Hello? How vulgar how no grade, just like the mouth with a penang, can not vomit breath, absolutely need to be corrected.

As a result, after changing to Hello, Gao Gong found that the application had to be restarted to take effect. It took more than ten seconds for the system to dawdle into life.

Some things are good over time, but intolerable for this kind of code debugging scenario.

Gao Gong spit out his mouth and thought, it’s time to develop a tool to speed up debugging.

That’s where SBDT comes in.

spring-boot-devtools

SpringBoot, StringBuilder, StringBuffer SpringBoot, in particular, is very handy thanks to its Autoconfig, which regulates development by convention.

The problem is that SpringBoot loads so many jars that each boot takes too long. For the SpringBoot service, spring-boot-DevTools is a timely shower of rain for students staring blandishly at the reboot screen.

It’s been around for a long time, but I find that in real projects, people don’t use it much. But it’s very simple to use.

Just add the following JAR package to the project’s POM file to get the second-level service reloads (hot deployment).

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-devtools</artifactId>
	<scope>runtime</scope>
	<optional>true</optional>
</dependency>
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Since the default value in the starter file is true, the following configuration in YML is not required.

spring:
  devtools:
    restart:
      enabled: true
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So let’s verify that. It’s coming fast.

Create a new simple controller and export Halloworld.

@Controller
public class DemoController {
    @GetMapping("/test")
    @ResponseBody
    public String test(a){
        return "halloworld"; }}Copy the code

Modify the code to change hallo to Hello. The console will start scrolling out the log and loading the project code. At this point, I visit the browser and see that our changes have taken effect.

# log display
Started MbyeApplication in1.731 seconds (JVM is runningfor 51.115)
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The console also outputs the restart time, which takes less than 2 seconds, which is pretty fast.

In order to trigger compilation in real time after code modification, you need to do the following configuration in IDEA. If this configuration does not work, manually click Build (note not rebuild).

Why is hot deployment reloading so fast? Because it doesn’t restart the entire application, it just restarts our application code.

By configuring the meta-INF /spring-devtools.properties file, you can specify that third-party JAR packages are loaded on each reboot. But it’s rare. Of course, include and exclude are included as shown in the following example.

restart.exclude.somejar=/somejar-[\\w-]+\.jar
restart.include.ajar=/ajar-[\\w-]+\.jar
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I noticed an interesting thing. When we start with IDEA, the console output looks like this.

The 21:33:59 2020-09-18. 4635-495 the INFO/restartedMain C.G.J avarunfast. Mbye. MbyeApplication: Starting MbyeApplication on LYCYs-MacBook-Pro.local with PID 4635 (/target/classes started by xjjdogin/ Users/xjjdog/codes / 21:33:59 javarunfast/mbye) 2020-09-18. 4635-495 the INFO [restartedMain] c.g.javarunfast.mbye.MbyeApplication : No active profileset, falling back to default profiles: Default 21:34:00 2020-09-18. 4635-355 the INFO [restartedMain] S.D.R.C.R epositoryConfigurationDelegate: Multiple Spring Data modules found, entering strict repository configuration mode! The 2020-09-18 21:34:00. 4635-355 the INFO [restartedMain]. S.D.R.C.R epositoryConfigurationDelegate: Bootstrapping Spring Data Elasticsearch repositoriesinThe DEFAULT mode. The 2020-09-18 21:34:00. 4635-357 the INFO [restartedMain] S.D.R.C.R epositoryConfigurationDelegate: Finished Spring Data repository scanningin1ms. Found 0 Elasticsearch repository interfaces. 2020-09-18 21:35:00.362 INFO 4635 - [restartedMain] .s.d.r.c.RepositoryConfigurationDelegate : Multiple Spring Data modules found, entering strict repository configuration mode! The 2020-09-18 21:34:00. 4635-362 the INFO [restartedMain]. S.D.R.C.R epositoryConfigurationDelegate: Bootstrapping Spring Data Reactive Elasticsearch repositoriesin DEFAULT mode.
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The starting thread inside is restartedMain. But when we start using Java -jar *jar, the main process is not restartedMain, but main.

This is because it doesn’t make sense to have DevTools on in an online environment.

That’s for now, because it hits you in the face.

More functions

To see what DevTools does, take a look at its source directory structure.

Filewatch and Classpath, of course, can be hot booted by listening for file changes. The principle is to use a separate ClassLoader (specifically RestartClassLoader) to complete the load replacement.

Look at this section of the code to get a better understanding of Java class loaders.

LiveReload

Next up is the Livereload feature.

LiveReload is used a lot when you’re doing front-end development.

Devtools also starts a LiveReload Server in the background, and the browser maintains a long connection to the Server. When a backend resource changes, the browser will be notified to refresh the Server for hot deployment.

Here is the Remote Live Reload plugin for Chrome. Install to have this cool feature.

https://chrome.google.com/webstore/detail/remotelivereload/jlppknnillhjgiengoigajegdpieppei?hl=en-GB
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Remote deployment

This is much more interesting. We mentioned above that devTools is meaningless in an online environment, so let’s face it now.

You may have low performance on your own machine and have your code run remotely while you develop your code locally. At this point, remote hot deployment can be used.

There are a few steps to turn this feature on.

Step 1.

You need to make the following changes to the spring-boot-Maven-plugin in POM.xml.

<plugin>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-maven-plugin</artifactId>
	<configuration>
		<excludeDevtools>false</excludeDevtools>
	</configuration>
</plugin>
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Step 2.

Set a key in YML to connect the server to the debugger.

spring:
  devtools:
    remote:
      secret: test
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Step 3.

Package the SB service as a JAR and start it.

mvn -Dmaven.test.skip=true -Pdev package

java -jar -Xdebug  \
-Xrunjdwp:server=y,transport=dt_socket,suspend= n \ mbye - 0.0.1 - the SNAPSHOT. The jarCopy the code

As you can see, we added a lot of parameters to boot, which is what it means to turn on remote molesting.

Step 4.

Edit a Java file in local IDEA and insert our server address (same as the application address) into the startup variable.

import org.springframework.boot.devtools.RemoteSpringApplication;

/ * * *@date2020/09/19 * /
public class Remote {
    public static void main(String[] args) {
        RemoteSpringApplication.main(new String[]{"http://localhost:8080"}); }}Copy the code

Step 5.

Verify. Edit any file you want to see and click Build.

Below is a screenshot of the IDEA development side.

The following is a screenshot of the server. You can see that the service has been reloaded, but very quickly.

 Listening forremote restart updates on /.~~spring-boot! ~/restart Started MbyeApplicationin1.961 seconds (JVM is runningfor 249.452)
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Access the Web page and find that the code has been uploaded successfully.

End

In fact, spring-boot-DevTools is not the most powerful. Because it reloads the project’s class file each time using ClassLoader. If your project has too many files, it will be slow.

There’s an even better tool called JRebel, which is really a great tool for development. Even better, it works with any Java project, not just SpringBoot projects. It’s a bit heavy, though, and there’s a charge. We’ll talk about it next time, or maybe you’ve already tried it out.

Xjjdog is a public account that doesn’t allow programmers to get sidetracked. Focus on infrastructure and Linux. Ten years architecture, ten billion daily flow, and you discuss the world of high concurrency, give you a different taste.