Java JDK新特性探究-精简的JRE

墨蓝 2022-12-13 01:28 231阅读 0赞

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2VqaW54aWFu_size_16_color_FFFFFF_t_70][]

java8有个很好的特性，即JEP161([http://openjdk.java.net/jeps/161][http_openjdk.java.net_jeps_161] ),该特性定义了Java SE平台规范的一些子集，使java应用程序不需要整个JRE平台即可部署和运行在小型设备上。开发人员可以基于目标硬件的可用资源选择一个合适的JRE运行环境。

# 好处 #

1.更小的Java环境需要更少的计算资源。

2.一个较小的运行时环境可以更好的优化性能和启动时间。

3.消除未使用的代码从安全的角度总是好的。

4.这些打包的应用程序可以下载速度更快。

# 概念 #

紧凑的JRE分3种，分别是[compact1][]、[compact2][]、[compact3][]，他们的关系是compact1<compact2<compact3,他们包含的API如下图所示

![3631a31004177aa79ac8cdd1d49b6c42.png][]

# 使用javac根据profile编译应用程序 #

javac –bootclasspath, or javac –profile <profile>

如果不符合compact的api，则报错。

$ javac -profile compact2 Test.java
    Test.java:7: error: ThreadMXBean is not available in profile 'compact2'
     ThreadMXBean bean = ManagementFactory.getThreadMXBean();
     ^
    Test.java:7: error: ManagementFactory is not available in profile 'compact2'
     ThreadMXBean bean = ManagementFactory.getThreadMXBean();
                         ^
    2 errors

# 使用工具开发的效果 #

![ba4353bcee8b3ea5b3bf1502d7449c5f.png][]

# JPEDS工具使用 #

java8新增一个工具，用来分析应用程序所依赖的profile，有三个参数比较常用 -p，-v，-r

import java.util.Set;
    import java.util.HashSet;
    
    public class Deps {
      public static void main(String[] args) {
        System.out.println(Math.random());
        Set<String> set = new HashSet<>();
      }
    }

************** PROFILE ********************
    jdeps -P Deps.class 
    Deps.class -> /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/rt.jar
       <unnamed> (Deps.class)
          -> java.io                                            compact1
          -> java.lang                                          compact1
          -> java.util                                          compact1
    
    ************** VERBOSE ********************
    jdeps -v Deps.class 
    Deps.class -> /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/rt.jar
       Deps (Deps.class)
          -> java.io.PrintStream                                
          -> java.lang.Math                                     
          -> java.lang.Object                                   
          -> java.lang.String                                   
          -> java.lang.System                                   
          -> java.util.HashSet  
    
    ************** RECURSIVE ********************
    jdeps -R Deps.class 
    Deps.class -> /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/rt.jar
       <unnamed> (Deps.class)
          -> java.io                                            
          -> java.lang                                          
          -> java.util                                          
    /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/jce.jar -> /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/rt.jar
       javax.crypto (jce.jar)
          -> java.io                                            
          -> java.lang                                          
          -> java.lang.reflect                                  
          -> java.net                                           
          -> java.nio                                           
          -> java.security                                      
          -> java.security.cert                                 
          -> java.security.spec                                 
          -> java.util                                          
          -> java.util.concurrent                               
          -> java.util.jar                                      
          -> java.util.regex                                    
          -> java.util.zip                                      
          -> javax.security.auth                                
          -> sun.security.jca                                   JDK internal API (rt.jar)
          -> sun.security.util                                  JDK internal API (rt.jar)
          -> sun.security.validator                             JDK internal API (rt.jar)
       javax.crypto.interfaces (jce.jar)
          -> java.lang                                          
          -> java.math                                          
          -> java.security                                      
       javax.crypto.spec (jce.jar)
          -> java.lang                                          
          -> java.math                                          
          -> java.security.spec                                 
          -> java.util                                          
    /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/rt.jar -> /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/jce.jar
       java.security (rt.jar)
          -> javax.crypto                                       JDK internal API (jce.jar)
       sun.security.util (rt.jar)
          -> javax.crypto                                       JDK internal API (jce.jar)
          -> javax.crypto.interfaces                            JDK internal API (jce.jar)
          -> javax.crypto.spec                                  JDK internal API (jce.jar)

# 在linux上构建profile #

$ hg clone http://hg.openjdk.java.net/jdk8/jdk8/
    $ cd jdk8
    $ make images profiles : 
    ## Finished profiles (build time 00:00:27)
    ----- Build times -------
    Start 2013-03-17 14:47:35
    End 2013-03-17 14:58:26
    00:00:25 corba
    00:00:15 demos
    00:01:50 hotspot
    00:00:24 images
    00:00:21 jaxp
    00:00:31 jaxws
    00:05:37 jdk
    00:00:43 langtools
    00:00:18 nashorn
    00:00:27 profiles
    00:10:51 TOTAL
    -------------------------
    Finished building Java(TM) for target 'images profiles'
    $ cd images
    $ ls -d *image
    j2re-compact1-image j2re-compact2-image j2re-compact3-image j2re-image j2sdk-image

# 编译后compact大致的占用空间  #

![e2d4cc3963c629eeaa04e775468a1694.png][]

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[http_openjdk.java.net_jeps_161]: http://openjdk.java.net/jeps/161
[compact1]: https://docs.oracle.com/javase/8/docs/api/compact1-summary.html
[compact2]: https://docs.oracle.com/javase/8/docs/api/compact2-summary.html
[compact3]: https://docs.oracle.com/javase/8/docs/api/compact3-summary.html
[3631a31004177aa79ac8cdd1d49b6c42.png]: /images/20221123/b0f5e892432f4c5183866e3ad801f736.png
[ba4353bcee8b3ea5b3bf1502d7449c5f.png]: /images/20221123/6b58c0e3fcd64b09a2e12b0f8d267f0d.png
[e2d4cc3963c629eeaa04e775468a1694.png]: /images/20221123/36595e130e204ba588b23113907f0844.png