JAVA-000

返回Week_01主页

作业要求

1、自己写一个简单的 Hello.java，里面需要涉及基本类型，四则运行，if 和 for，然后 自己分析一下对应的字节码，有问题群里讨论。

演示Java Class文件的字节码(bytecode)，以及 javac 和 javap 工具的使用

• Hello World
• 四则运算
• if 判断
• for 循环

Reference:

• View Bytecode of a Class File in Java
  介绍了javac, javap 的基本用法，以及一些 bytecode 的小工具
• Introduction to Java Bytecode
  介绍了 bytecode 的基本概念，方便入门
• The Java 11 Virtual Machine Instruction Set
  Java 11 虚拟机指令集的官方文档。示例使用的java 11，文档中也会多次引用Java 11的官方文档。
• The Java 8 Virtual Machine Instruction Set
  Java 8 虚拟机指令集的官方文档。仅供对比参考。

  javac

  Reads Java class and interface definitions and compiles them into bytecode and class files. javac是用来编译 .java 文件的。

  Usage: javac where possible options include: @ Read options and filenames from file -Akey[=value] Options to pass to annotation processors --add-modules (,)* Root modules to resolve in addition to the initial modules, or all modules on the module path if is ALL-MODULE-PATH. --boot-class-path , -bootclasspath Override location of bootstrap class files --class-path , -classpath , -cp Specify where to find user class files and annotation processors -d Specify where to place generated class files -deprecation Output source locations where deprecated APIs are used --enable-preview Enable preview language features. To be used in conjunction with either -source or --release. -encoding Specify character encoding used by source files -endorseddirs Override location of endorsed standards path -extdirs Override location of installed extensions -g Generate all debugging info -g:{lines,vars,source} Generate only some debugging info -g:none Generate no debugging info -h Specify where to place generated native header files --help, -help, -? Print this help message --help-extra, -X Print help on extra options -implicit:{none,class} Specify whether or not to generate class files for implicitly referenced files -J Pass directly to the runtime system --limit-modules (,)* Limit the universe of observable modules --module , -m Compile only the specified module, check timestamps --module-path , -p Specify where to find application modules --module-source-path Specify where to find input source files for multiple modules --module-version Specify version of modules that are being compiled -nowarn Generate no warnings -parameters Generate metadata for reflection on method parameters -proc:{none,only} Control whether annotation processing and/or compilation is done. -processor [,,...] Names of the annotation processors to run; bypasses default discovery process --processor-module-path Specify a module path where to find annotation processors --processor-path , -processorpath Specify where to find annotation processors -profile Check that API used is available in the specified profile --release Compile for a specific VM version. Supported targets: 6, 7, 8, 9, 10, 11 -s Specify where to place generated source files -source Provide source compatibility with specified release --source-path , -sourcepath Specify where to find input source files --system |none Override location of system modules -target Generate class files for specific VM version --upgrade-module-path Override location of upgradeable modules -verbose Output messages about what the compiler is doing --version, -version Version information -Werror Terminate compilation if warnings occur

这里介绍一个最基本的用法，以 helloworld 为例，按照 IntelliJ IDEA 的文件组织结构编译java文件为class文件：

cd "C:\Data\Code\GitHub\GeekbangJavaTrainingCamp\Week_01\Bytecode\bytecode"
javac "src\main\java\org\example\helloworld\HelloByteCode.java" -d "target\classes"

javap

javap (Java Disassembler): Disassembles one or more class files.
javap是JDK提供的一个命令行工具,javap能对给定的class文件提供的字节代码进行反编译。 通过它，可以对照源代码和字节码，从而了解很多编译器内部的工作,对更深入地理解如何提高程序执行的效率等问题有极大的帮助。

Usage: javap <options> <classes>
where possible options include:
  -? -h --help -help               Print this help message
  -version                         Version information
  -v  -verbose                     Print additional information
  -l                               Print line number and local variable tables
  -public                          Show only public classes and members
  -protected                       Show protected/public classes and members
  -package                         Show package/protected/public classes and members (default)
  -p  -private                     Show all classes and members
  -c                               Disassemble the code
  -s                               Print internal type signatures
  -sysinfo                         Show system info (path, size, date, MD5 hash) of class being processed
  -constants                       Show final constants
  --module <module>, -m <module>   Specify module containing classes to be disassembled
  --module-path <path>             Specify where to find application modules
  --system <jdk>                   Specify where to find system modules
  --class-path <path>              Specify where to find user class files
  -classpath <path>                Specify where to find user class files
  -cp <path>                       Specify where to find user class files
  -bootclasspath <path>            Override location of bootstrap class files

GNU-style options may use = instead of whitespace to separate the name of an option
from its value.

Each class to be shown may be specified by a filename, a URL, or by its fully
qualified class name. Examples:
   path/to/MyClass.class
   jar:file:///path/to/MyJar.jar!/mypkg/MyClass.class
   java.lang.Object