从本机的jdk目录下载源码
如图所示：
打开任何一个 JDK8 项目查看 JDK8源码目录，这里 JDK home path 即 JDK 的安装目录，跳转到 JDK8 的这个目录路径：/Users/vitah/.sdkman/candidates/java/8.0.312-tem。
> cd /Users/vitah/.sdkman/candidates/java/8.0.312-tem
> tree -L 1
.
├── ASSEMBLY_EXCEPTION
├── LICENSE
├── NOTICE
├── THIRD_PARTY_README
├── bin
├── bundle
├── include
├── jre
├── lib
├── man
├── release
├── sample
└── src.zip
7 directories, 6 files
可以看到目录下有一个 src.zip 的压缩包，这个即是 JDK8 的源代码压缩包。
新建项目导入源代码
新建名为 jdk8-analyse 的空目录，将 src.zip 解压代码导入当前文件夹，结构如下：
jdk8-analyse
└── src
├── com
├── java
├── javax
├── jdk
├── launcher
├── org
└── sun
8 directories
导入后，直接用 Intellij-Idea 打开项目 jdk8-analyse。
编译错误处理
java: Compilation failed: internal java compiler error
编译时，出现如下错误：
java: Compilation failed: internal java compiler error
调整 Idea 配置：Preferences → Build, Execution, Deployment → Compiler，修改 Shared build process heap size 的大小，将700改为3000。
snmp 相关类缺失
提示 SnmpOid 相关类缺失，具体错误如下：
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/com/sun/jmx/snmp/daemon/SnmpAdaptorServer.java:53:24
java: cannot find symbol
symbol: class SnmpOid
location: package com.sun.jmx.snmp
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/com/sun/jmx/snmp/daemon/SnmpAdaptorServer.java:55:24
java: cannot find symbol
symbol: class SnmpPduPacket
location: package com.sun.jmx.snmp
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/com/sun/jmx/snmp/daemon/SnmpAdaptorServer.java:58:24
java: cannot find symbol
symbol: class SnmpTimeticks
location: package com.sun.jmx.snmp
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/com/sun/jmx/snmp/daemon/SnmpAdaptorServer.java:59:24
java: cannot find symbol
symbol: class SnmpVarBind
location: package com.sun.jmx.snmp
这是因为 OpenJDK 采用 GPL V2协议，Oracle JDK 采用 JRL 协议，两者虽然都是开放源代码的，但是 GPL V2 允许商业使用，而 JRL 只允许个人研究使用。SUN JDK 的一部分源代码因为产权的问题无法开放给 OpenJDK 使用，其中最主要的部分就是 jmx 中的可选元件 snmp 部分的代码。
解决方案：下载 snmp 相关缺失类，直接复制到对应目录中。
DuctusRenderingEngine类相关包缺失
具体错误内容：
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:39:17
java: package sun.dc.pr does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:40:17
java: package sun.dc.pr does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:41:17
java: package sun.dc.pr does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:42:17
java: package sun.dc.pr does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:43:19
java: package sun.dc.path does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:44:19
java: package sun.dc.path does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:45:19
java: package sun.dc.path does not exist
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:174:54
java: cannot find symbol
symbol: class PathConsumer
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:176:16
java: cannot find symbol
symbol: class PathException
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:360:20
java: cannot find symbol
symbol: class Rasterizer
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:362:32
java: cannot find symbol
symbol: class Rasterizer
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:372:52
java: cannot find symbol
symbol: class Rasterizer
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:720:31
java: cannot find symbol
symbol: class PathConsumer
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:770:42
java: cannot find symbol
symbol: class PathConsumer
location: class sun.dc.DuctusRenderingEngine
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:787:16
java: cannot find symbol
symbol: class PathConsumer
location: class sun.dc.DuctusRenderingEngine.FillAdapter
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:826:30
java: cannot find symbol
symbol: class FastPathProducer
location: class sun.dc.DuctusRenderingEngine.FillAdapter
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/dc/DuctusRenderingEngine.java:827:20
java: cannot find symbol
symbol: class PathException
location: class sun.dc.DuctusRenderingEngine.FillAdapter
这里直接删除 sun/dc/DuctusRenderingEngine 类 。
sun.nio.fs包下相关类缺失
具体错误内容：
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/nio/fs/SolarisUserDefinedFileAttributeView.java:36:25
java: cannot find symbol
symbol: class SolarisConstants
location: package sun.nio.fs
解决方式：直接删除 sun.nio.fs 整个包。
XEvent类缺失
具体错误：
/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src/sun/awt/X11/XComponentPeer.java:1242:39
java: cannot find symbol
symbol: class XEvent
location: class sun.awt.X11.XComponentPeer
解决方式：直接删除 sun/awt/X11 包。
调试jdk源码，自由注释修改
将JDK换成当前项目
新建测试类 ArrayListTest：
public class ArrayListTest {
public static void main(String[] args) {
List list = new ArrayList();
list.add(1);
System.out.println(list.size());
}
}
跳转到 ArrayList 的构造函数，发现 ArrayList.java 文件还是只读的，这是因为现在看到的 ArrayList.java 代码还是本机安装的 jdk8，而不是刚才导入的源代码。
Idea → File → Project Structrue，如图所示：
可以看到，上述图中使用的 jdk 源代码仍然是系统安装的。这里将其切换为当前项目的 jdk 代码。
选择新增 SDK，JDK home path 和原来的保持一致，这里配置成 /Users/vitah/.sdkman/candidates/java/8.0.312-tem，修改 Sourcepath，选择当前项目的 src 目录，这里选择 /Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/src：
点击 Apply 保存，然后修改当前项目的 sdk 为 java8-analyse，新建测试类 ArrayListTest ：
public class ArrayListTest {
public static void main(String[] args) {
List list = new ArrayList();
list.add(1);
System.out.println(list.size());
}
}
并且修改，ArrayList 的构造函数如下图：
执行输出结果如下：
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=58732:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/charsets.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/cldrdata.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/dnsns.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/jaccess.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/localedata.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/nashorn.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/sunec.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/sunjce_provider.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/sunpkcs11.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/zipfs.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jce.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jfr.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jsse.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/management-agent.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/resources.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/rt.jar:/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/out/production/jdk8-analyse vitahlin.ArrayListTest
1
Process finished with exit code 0
发现在构造函数里面的 println 函数并不能打印。
使jdk中自己修改的代码能够执行
如何使 JDK 代码中的 println 函数生效，可以参考这个链接：https://www.cnblogs.com/grey-wolf/p/12817615.html#_label4
这个值的生成规则可以参考代码（运行时，修改 myPath 为自己本机路径即可）：
public static void main(String[] args) {
String pathTotal = System.getProperty("sun.boot.class.path");
String[] paths = pathTotal.split(":");
Arrays.stream(paths).forEach(System.out::println);
String myPath = "/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/out/production/jdk8-analyse";
String vmOption = "-Dsun.boot.class.path=" + myPath + ":" + pathTotal;
System.out.println("\nCopy the result and add to vm option:");
System.out.println(vmOption);
}
执行结果为：
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=59625:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/charsets.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/cldrdata.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/dnsns.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/jaccess.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/localedata.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/nashorn.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/sunec.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/sunjce_provider.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/sunpkcs11.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/ext/zipfs.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jce.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jfr.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jsse.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/management-agent.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/resources.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/rt.jar:/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/out/production/jdk8-analyse vitahlin.GenerateVmOption
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/resources.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/rt.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/sunrsasign.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jsse.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jce.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/charsets.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jfr.jar
/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/classes
Copy the result and add to vm option:
-Dsun.boot.class.path=/Users/vitah/Downloads/dev/vitah/jarvan/jdk8-analyse/out/production/jdk8-analyse:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/resources.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/rt.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/sunrsasign.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jsse.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jce.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/charsets.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/lib/jfr.jar:/Users/vitah/.sdkman/candidates/java/8.0.312-tem/jre/classes
Process finished with exit code 0
复制12行的内容将其配置为 **ArrayListTest** 的 **vm option**
然后再执行，会看到现在执行结果如下：
This is my ArrayList
This is my ArrayList
This is my ArrayList
This is my ArrayList
This is my ArrayList
This is my ArrayList
This is my ArrayList
This is my ArrayList
1
Process finished with exit code 0
这样，就可以给 JDK 自由增加注释修改源代码了。