依稀记得自己第一次编译*.java文件，第一次运行*.class文件。但是六七年过去了，现在运行java写的程序更多的是用tomcat这种web容器。最近有个小需求，写一个监控zookeeper集群的报警器，当发现集群中节点发生变化时，发出邮件和短信通知运维人员。如果这么一个功能也写成一个web项目放到tomcat里，就有些杀鸡用牛刀了。于是就写了一个jar项目，用 java -jar 运行。占用资源少不说，部署启动很简单，也不占用访问端口。但也遇到了很多问题，才发现自己对java命令还是一知半解。写篇文章，一点一滴的记录一下如何使用java命令。
JDK下的命令
先来看看JDK下有哪些命令。
> cd $JAVA_HOME/bin > pwd /Library/Java/JavaVirtualMachines/jdk1.7.0_75.jdk/Contents/Home/bin > ls appletviewer javafxpackager jmc native2ascii servertool apt javah jps orbd tnameserv extcheck javap jrunscript pack200 unpack200 idlj jcmd jsadebugd policytool wsgen jar jconsole jstack rmic wsimport jarsigner jdb jstat rmid xjc java jhat jstatd rmiregistry javac jinfo jvisualvm schemagen javadoc jmap keytool serialver
目测我只会javac、java、javadoc、jar其他的就不知道了，木有关系，从现在开始，一个一个百度谷歌弄都了就好。我相信弄懂后不出一个月我就又会忘记，学习本身就是这样，学的多忘的多，但为了保持学习能力一定要不停的学^_^。
javac
javac主要是将*.java文件编译成*.class文件。*.class文件就可以放到java虚拟机里运行了。其实编译的过程就是将人能看懂的东西转换成机器能看懂的东西。
敲一下命令看看有啥操作参数.
> javac -help 用法: javac <options> <source files> 其中, 可能的选项包括: -g 生成所有调试信息 -g:none 不生成任何调试信息 -g:{lines,vars,source} 只生成某些调试信息 -nowarn 不生成任何警告 -verbose 输出有关编译器正在执行的操作的消息 -deprecation 输出使用已过时的 API 的源位置 -classpath <路径> 指定查找用户类文件和注释处理程序的位置 -cp <路径> 指定查找用户类文件和注释处理程序的位置 -sourcepath <路径> 指定查找输入源文件的位置 -bootclasspath <路径> 覆盖引导类文件的位置 -extdirs <目录> 覆盖所安装扩展的位置 -endorseddirs <目录> 覆盖签名的标准路径的位置 -proc:{none,only} 控制是否执行注释处理和/或编译。 -processor <class1>[,<class2>,<class3>...] 要运行的注释处理程序的名称; 绕过默认的搜索进程 -processorpath <路径> 指定查找注释处理程序的位置 -d <目录> 指定放置生成的类文件的位置 -s <目录> 指定放置生成的源文件的位置 -implicit:{none,class} 指定是否为隐式引用文件生成类文件 -encoding <编码> 指定源文件使用的字符编码 -source <发行版> 提供与指定发行版的源兼容性 -target <发行版> 生成特定 VM 版本的类文件 -version 版本信息 -help 输出标准选项的提要 -A关键字[=值] 传递给注释处理程序的选项 -X 输出非标准选项的提要 -J<标记> 直接将 <标记> 传递给运行时系统 -Werror 出现警告时终止编译 @<文件名> 从文件读取选项和文件名
再来看看扩展的操作有哪些。
> javac -X -Xlint 启用建议的警告 -Xlint:{all,cast,classfile,deprecation,dep-ann,divzero,empty,fallthrough,finally,options,overrides,path,processing,rawtypes,serial,static,try,unchecked,varargs,-cast,-classfile,-deprecation,-dep-ann,-divzero,-empty,-fallthrough,-finally,-options,-overrides,-path,-processing,-rawtypes,-serial,-static,-try,-unchecked,-varargs,none} 启用或禁用特定的警告 -Xbootclasspath/p:<路径> 置于引导类路径之前 -Xbootclasspath/a:<路径> 置于引导类路径之后 -Xbootclasspath:<路径> 覆盖引导类文件的位置 -Djava.ext.dirs=<目录> 覆盖所安装扩展的位置 -Djava.endorsed.dirs=<目录> 覆盖签名的标准路径的位置 -Xmaxerrs <编号> 设置要输出的错误的最大数目 -Xmaxwarns <编号> 设置要输出的警告的最大数目 -Xstdout <文件名> 重定向标准输出 -Xprint 输出指定类型的文本表示 -XprintRounds 输出有关注释处理循环的信息 -XprintProcessorInfo 输出有关请求处理程序处理哪些注释的信息 -Xprefer:{source,newer} 指定读取文件, 当同时找到隐式编译类的源文件和类文件时 -Xpkginfo:{always,legacy,nonempty} 指定 package-info 文件的处理 这些选项都是非标准选项, 如有更改, 恕不另行通知。
说实话，看着挺枯燥，里面知道怎么用的还是知道，不知道的，看完还是不知道。我们还是来看看man怎么说。
>man javac Name javac - Java programming language compiler SYNOPSIS javac [ options ] [ sourcefiles ] [ classes ] [ @argfiles ] Arguments may be in any order. options Command-line options. sourcefiles One or more source files to be compiled (such as MyClass.java). classes One or more classes to be processed for annotations (such as MyPackage.MyClass). @argfiles One or more files that lists options and source files. The -J options are not allowed in these files. DESCRIPTION The javac tool reads class and interface definitions, written in the Java programming language, and compiles them into bytecode class files. It can also process annotations in Java source files and classes. There are two ways to pass source code file names to javac: o For a small number of source files, simply list the file names on the command line. o For a large number of source files, list the file names in a file, separated by blanks or line breaks. Then use the list file name on the javac command line, preceded by an @ character. Source code file names must have .java suffixes, class file names must have .class suffixes, and both source and class files must have root names that identify the class. For example, a class called MyClass would be written in a source file called MyClass.java and compiled into a bytecode class file called MyClass.class. Inner class definitions produce additional class files. These class files have names combining the inner and outer class names, such as MyClass$MyInnerClass.class. You should arrange source files in a directory tree that reflects their package tree. For example, if you keep all your source files in /workspace, the source code for com.mysoft.mypack.MyClass should be in /workspace/com/mysoft/mypack/MyClass.java. By default, the compiler puts each class file in the same directory as its source file. You can specify a separate destination directory with -d (see Options, below). OPTIONS The compiler has a set of standard options that are supported on the current development environment and will be supported in future releases. An additional set of non-standard options are specific to the current virtual machine and compiler implementations and are subject to change in the future. Non-standard options begin with -X. Standard Options -Akey[=value] Options to pass to annotation processors. These are not inter- preted by javac directly, but are made available for use by indi- vidual processors. key should be one or more identifiers sepa- rated by ".". -cp path or -classpath path Specify where to find user class files, and (optionally) annota- tion processors and source files. This class path overrides the user class path in the CLASSPATH environment variable. If neither CLASSPATH, -cp nor -classpath is specified, the user class path consists of the current directory. See Setting the Class Path for more details. >If the -sourcepath option is not specified, the user class path is also searched for source files. If the -processorpath option is not specified, the class path is also searched for annotation processors. -Djava.ext.dirs=directories Override the location of installed extensions. -Djava.endorsed.dirs=directories Override the location of endorsed standards path. -d directory Set the destination directory for class files. The directory must already exist; javac will not create it. If a class is part of a package, javac puts the class file in a subdirectory reflecting the package name, creating directories as needed. For example, if you specify -d /home/myclasses and the class is called com.mypackage.MyClass, then the class file is called /home/myclasses/com/mypackage/MyClass.class. If -d is not specified, javac puts each class files in the same directory as the source file from which it was generated. Note: The directory specified by -d is not automatically added to your user class path. -deprecation Show a description of each use or override of a deprecated member or class. Without -deprecation, javac shows a summary of the source files that use or override deprecated members or classes. -deprecation is shorthand for -Xlint:deprecation. -encoding encoding Set the source file encoding name, such as EUC-JP and UTF-8. If -encoding is not specified, the platform default converter is used. -endorseddirs directories Override the location of endorsed standards path. -extdirs directories Overrides the location of the ext directory. The directories variable is a colon-separated list of directories. Each JAR ar- chive in the specified directories is searched for class files. All JAR archives found are automatically part of the class path. If you are cross-compiling (compiling classes against bootstrap and extension classes of a different Java platform implementa- tion), this option specifies the directories that contain the extension classes. See Cross-Compilation Options for more infor- mation. -g Generate all debugging information, including local variables. By default, only line number and source file information is gener- ated. -g:none Do not generate any debugging information. -g:{keyword list} Generate only some kinds of debugging information, specified by a comma separated list of keywords. Valid keywords are: source Source file debugging information lines Line number debugging information vars Local variable debugging information -help Print a synopsis of standard options. -implicit:{class,none} Controls the generation of class files for implicitly loaded source files. To automatically generate class files, use -implicit:class. To suppress class file generation, use -implicit:none. If this option is not specified, the default is to automatically generate class files. In this case, the compiler will issue a warning if any such class files are generated when also doing annotation processing. The warning will not be issued if this option is set explicitly. See Searching For Types. -Joption Pass option to the java launcher called by javac. For example, -J-Xms48m sets the startup memory to 48 megabytes. It is a common convention for -J to pass options to the underlying VM executing applications written in Java. Note: CLASSPATH, -classpath, -bootclasspath, and -extdirs do not specify the classes used to run javac. Fiddling with the imple- mentation of the compiler in this way is usually pointless and always risky. If you do need to do this, use the -J option to pass through options to the underlying java launcher. -nowarn Disable warning messages. This has the same meaning as -Xlint:none. -proc: {none,only} Controls whether annotation processing and/or compilation is done. -proc:none means that compilation takes place without anno- tation processing. -proc:only means that only annotation process- ing is done, without any subsequent compilation. -processor class1[,class2,class3...] Names of the annotation processors to run. This bypasses the default discovery process. -processorpath path Specify where to find annotation processors; if this option is not used, the class path will be searched for processors. -s dir Specify the directory where to place generated source files. The directory must already exist; javac will not create it. If a class is part of a package, the compiler puts the source file in a subdirectory reflecting the package name, creating directories as needed. For example, if you specify -s /home/mysrc and the class is called com.mypackage.MyClass, then the source file will be placed in /home/mysrc/com/mypackage/MyClass.java. -source release Specifies the version of source code accepted. The following val- ues for release are allowed: 1.3 The compiler does not support assertions, generics, or other language features introduced after Java SE 1.3. 1.4 The compiler accepts code containing assertions, which were introduced in Java SE 1.4. 1.5 The compiler accepts code containing generics and other lan- guage features introduced in Java SE 5. 5 Synonym for 1.5. 1.6 No language changes were introduced in Java SE 6. However, encoding errors in source files are now reported as errors instead of warnings as in previous releases of Java SE. 6 Synonym for 1.6. 1.7 This is the default value. The compiler accepts code with fea- tures introduced in Java SE 7. 7 Synonym for 1.7. -sourcepath sourcepath Specify the source code path to search for class or interface definitions. As with the user class path, source path entries are separated by colons (:) and can be directories, JAR archives, or ZIP archives. If packages are used, the local path name within the directory or archive must reflect the package name. Note: Classes found through the class path may be subject to automatic recompilation if their sources are also found. See Searching For Types. -verbose Verbose output. This includes information about each class loaded and each source file compiled. -version Print version information. -Werror Terminate compilation if warnings occur. -X Display information about non-standard options and exit. Cross-Compilation Options By default, classes are compiled against the bootstrap and extension classes of the platform that javac shipped with. But javac also sup- ports cross-compiling, where classes are compiled against a bootstrap and extension classes of a different Java platform implementation. It is important to use -bootclasspath and -extdirs when cross-compiling; see Cross-Compilation Example below. -target version Generate class files that target a specified version of the VM. Class files will run on the specified target and on later ver- sions, but not on earlier versions of the VM. Valid targets are 1.1, 1.2, 1.3, 1.4, 1.5 (also 5), 1.6 (also 6), and 1.7 (also 7). The default for -target depends on the value of -source: o If -source is not specified, the value of -target is 1.7 o If -source is 1.2, the value of -target is 1.4 o If -source is 1.3, the value of -target is 1.4 o If -source is 1.5, the value of -target is 1.7 o If -source is 1.6, the value of -target is 1.7 o For all other values of -source, the value of -target is the value of -source. -bootclasspath bootclasspath Cross-compile against the specified set of boot classes. As with the user class path, boot class path entries are separated by colons (:) and can be directories, JAR archives, or ZIP archives. Non-Standard Options -Xbootclasspath/p:path Prepend to the bootstrap class path. -Xbootclasspath/a:path Append to the bootstrap class path. -Xbootclasspath/:path Override location of bootstrap class files. -Xlint Enable all recommended warnings. In this release, enabling all available warnings is recommended. -Xlint:all Enable all recommended warnings. In this release, enabling all available warnings is recommended. -Xlint:none Disable all warnings. -Xlint:name Enable warning name. See the section Warnings That Can Be Enabled or Disabled with -Xlint Option for a list of warnings you can enable with this option. -Xlint:-name Disable warning name. See the section Warnings That Can Be Enabled or Disabled with -Xlint Option for a list of warnings you can disable with this option. -Xmaxerrs number Set the maximum number of errors to print. -Xmaxwarns number Set the maximum number of warnings to print. -Xstdout filename Send compiler messages to the named file. By default, compiler messages go to System.err. -Xprefer:{newer,source} Specify which file to read when both a source file and class file are found for a type. (See Searching For Types). If -Xpre- fer:newer is used, it reads the newer of the source or class file for a type (default). If the -Xprefer:source option is used, it reads source file. Use -Xprefer:source when you want to be sure that any annotation processors can access annotations declared with a retention policy of SOURCE. -Xpkginfo:{always,legacy,nonempty} Specify handling of package-info files -Xprint Print out textual representation of specified types for debugging purposes; perform neither annotation processing nor compilation. The format of the output may change. -XprintProcessorInfo Print information about which annotations a processor is asked to process. -XprintRounds Print information about initial and subsequent annotation pro- cessing rounds. Warnings That Can Be Enabled or Disabled with -Xlint Option Enable warning name with the option -Xlint:name, where name is one of the following warning names. Similarly, you can disable warning name with the option -Xlint:-name: cast Warn about unnecessary and redundant casts. For example: String s = (String)"Hello!" classfile Warn about issues related to classfile contents. deprecation Warn about use of deprecated items. For example: java.util.Date myDate = new java.util.Date(); int currentDay = myDate.getDay(); The method java.util.Date.getDay has been deprecated since JDK 1.1. dep-ann Warn about items that are documented with an @deprecated Javadoc comment, but do not have a @Deprecated annotation. For example: /** * @deprecated As of Java SE 7, replaced by {@link #newMethod()} */ public static void deprecatedMethood() { } public static void newMethod() { } divzero Warn about division by constant integer 0. For example: int divideByZero = 42 / 0; empty Warn about empty statements after if statements. For example: class E { void m() { if (true) ; } } fallthrough Check switch blocks for fall-through cases and provide a warning message for any that are found. Fall-through cases are cases in a switch block, other than the last case in the block, whose code does not include a break statement, allowing code execution to "fall through" from that case to the next case. For example, the code following the case 1 label in this switch block does not end with a break statement: switch (x) { case 1: System.out.println("1"); // No break statement here. case 2: System.out.println("2"); } If the -Xlint:fallthrough flag were used when compiling this code, the compiler would emit a warning about "possible fall-through into case," along with the line number of the case in question. finally Warn about finally clauses that cannot complete normally. For example: public static int m() { try { throw new NullPointerException(); } catch (NullPointerException e) { System.err.println("Caught NullPointerException."); return 1; } finally { return 0; } } The compiler generates a warning for finally block in this exam- ple. When this method is called, it returns a value of 0, not 1. A finally block always executes when the try block exits. In this example, if control is transferred to the catch, then the method exits. However, the finally block must be executed, so it is exe- cuted, even though control has already been transferred outside the method. options Warn about issues relating to the use of command line options. See Cross-Compilation Example for an example of this kind of warning. overrides Warn about issues regarding method overrides. For example, con- sider the following two classes: public class ClassWithVarargsMethod { void varargsMethod(String... s) { } } public class ClassWithOverridingMethod extends ClassWithVarargsMethod { @Override void varargsMethod(String[] s) { } } The compiler generates a warning similar to the following: warning: [override] varargsMethod(String[]) in ClassWithOverrid- ingMethod overrides varargsMethod(String...) in ClassWith- VarargsMethod; overriding method is missing '...' When the compiler encounters a varargs method, it translates the varargs formal parameter into an array. In the method ClassWith- VarargsMethod.varargsMethod, the compiler translates the varargs formal parameter String... s to the formal parameter String[] s, an array, which matches the formal parameter of the method Class- WithOverridingMethod.varargsMethod. Consequently, this example compiles. path Warn about invalid path elements and nonexistent path directories on the command line (with regards to the class path, the source path, and other paths). Such warnings cannot be suppressed with the @SuppressWarnings annotation. For example: javac -Xlint:path -classpath /nonexistentpath Example.java processing Warn about issues regarding annotation processing. The compiler generates this warning if you have a class that has an annota- tion, and you use an annotation processor that cannot handle that type of exception. For example, the following is a simple annota- tion processor: Source file AnnoProc.java: import java.util.*; import javax.annotation.processing.*; import javax.lang.model.*; import javax.lang.model.element.*; @SupportedAnnotationTypes("NotAnno") public class AnnoProc extends AbstractProcessor { public boolean process(Set<? extends TypeElement> elems, RoundEnvironment renv) { return true; } public SourceVersion getSupportedSourceVersion() { return SourceVersion.latest(); } } Source file AnnosWithoutProcessors.java: @interface Anno { } @Anno class AnnosWithoutProcessors { } The following commands compile the annotation processor AnnoProc, then run this annotation processor against the source file AnnosWithoutProcessors.java: % javac AnnoProc.java % javac -cp . -Xlint:processing -processor AnnoProc -proc:only AnnosWithoutProcessors.java When the compiler runs the annotation processor against the source file AnnosWithoutProcessors.java, it generates the follow- ing warning: warning: [processing] No processor claimed any of these annota- tions: Anno To resolve this issue, you can rename the annotation defined and used in the class AnnosWithoutProcessors from Anno to NotAnno. rawtypes Warn about unchecked operations on raw types. The following statement generates a rawtypes warning: void countElements(List l) { ... } The following does not generate a rawtypes warning: void countElements(List<?> l) { ... } List is a raw type. However, List<?> is a unbounded wildcard parameterized type. Because List is a parameterized interface, you should always specify its type argument. In this example, the List formal argument is specified with a unbounded wildcard (?) as its formal type parameter, which means that the countElements method can accept any instantiation of the List interface. serial Warn about missing serialVersionUID definitions on serializable classes. For example: public class PersistentTime implements Serializable { private Date time; public PersistentTime() { time = Calendar.getInstance().getTime(); } public Date getTime() { return time; } } The compiler generates the following warning: warning: [serial] serializable class PersistentTime has no defi- nition of serialVersionUID If a serializable class does not explicitly declare a field named serialVersionUID, then the serialization runtime will calculate a default serialVersionUID value for that class based on various aspects of the class, as described in the Java Object Serializa- tion Specification. However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values because the default process of computing serialVersionUID vales is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. Therefore, to guarantee a consistent serialVersionUID value across different Java compiler implementations, a serializable class must declare an explicit serialVersionUID value. static Warn about issues relating to use of statics. For example: class XLintStatic { static void m1() { } void m2() { this.m1(); } } The compiler generates the following warning: warning: [static] static method should be qualified by type name, XLintStatic, instead of by an expression To resolve this issue, you can call the static method m1 as fol- lows: XLintStatic.m1(); Alternatively, you can remove the static keyword from the decla- ration of the method m1. try Warn about issues relating to use of try blocks, including try-with-resources statements. For example, a warning is gener- ated for the following statement because the resource ac declared in the try statement is not used: try ( AutoCloseable ac = getResource() ) { // do nothing } unchecked Give more detail for unchecked conversion warnings that are man- dated by the Java Language Specification. For example: List l = new ArrayList<Number>(); List<String> ls = l; // unchecked warning During type erasure, the types ArrayList<Number> and List<String> become ArrayList and List, respectively. The variable ls has the parameterized type List<String>. When the List referenced by l is assigned to ls, the compiler generates an unchecked warning; the compiler is unable to determine at compile time, and moreover knows that the JVM will not be able to deter- mine at runtime, if l refers to a List<String> type; it does not. Consequently, heap pollution occurs. In detail, a heap pollution situation occurs when the List object l, whose static type is List<Number>, is assigned to another List object, ls, that has a different static type, List<String>. How- ever, the compiler still allows this assignment. It must allow this assignment to preserve backwards compatibility with versions of Java SE that do not support generics. Because of type erasure, List<Number> and List<String> both become List. Consequently, the compiler allows the assignment of the object l, which has a raw type of List, to the object ls. varargs Warn about unsafe usages of variable arguments (varargs) methods, in particular, those that contain non-reifiable arguments. For example: public class ArrayBuilder { public static <T> void addToList (List<T> listArg, T... elements) { for (T x : elements) { listArg.add(x); } } } The compiler generates the following warning for the definition of the method ArrayBuilder.addToList: warning: [varargs] Possible heap pollution from parameterized vararg type T When the compiler encounters a varargs method, it translates the varargs formal parameter into an array. However, the Java pro- gramming language does not permit the creation of arrays of parameterized types. In the method ArrayBuilder.addToList, the compiler translates the varargs formal parameter T... elements to the formal parameter T[] elements, an array. However, because of type erasure, the compiler converts the varargs formal parameter to Object[] elements. Consequently, there is a possibility of heap pollution. COMMAND LINE ARGUMENT FILES To shorten or simplify the javac command line, you can specify one or more files that themselves contain arguments to the javac command (except -J options). This enables you to create javac commands of any length on any operating system. An argument file can include javac options and source filenames in any combination. The arguments within a file can be space-separated or new- line-separated. If a filename contains embedded spaces, put the whole filename in double quotes. Filenames within an argument file are relative to the current direc- tory, not the location of the argument file. Wildcards (*) are not allowed in these lists (such as for specifying *.java). Use of the '@' character to recursively interpret files is not supported. The -J options are not supported because they are passed to the launcher, which does not support argument files. When executing javac, pass in the path and name of each argument file with the '@' leading character. When javac encounters an argument beginning with the character `@', it expands the contents of that file into the argument list. Example - Single Arg File You could use a single argument file named "argfile" to hold all javac arguments: % javac @argfile This argument file could contain the contents of both files shown in the next example. Example - Two Arg Files You can create two argument files -- one for the javac options and the other for the source filenames: (Notice the following lists have no line-continuation characters.) Create a file named "options" containing: -d classes -g -sourcepath /java/pubs/ws/1.3/src/share/classes Create a file named "classes" containing: MyClass1.java MyClass2.java MyClass3.java You would then run javac with: % javac @options @classes Example - Arg Files with Paths The argument files can have paths, but any filenames inside the files are relative to the current working directory (not path1 or path2): % javac @path1/options @path2/classes ANNOTATION PROCESSING javac provides direct support for annotation processing, superseding the need for the separate annotation processing tool, apt. The API for annotation processors is defined in the javax.annota- tion.processing and javax.lang.model packages and subpackages. Overview of annotation processing Unless annotation processing is disabled with the -proc:none option, the compiler searches for any annotation processors that are available. The search path can be specified with the -processorpath option; if it is not given, the user class path is used. Processors are located by means of service provider-configuration files named META-INF/ser- vices/javax.annotation.processing.Processor on the search path. Such files should contain the names of any annotation processors to be used, listed one per line. Alternatively, processors can be specified explic- itly, using the -processor option. After scanning the source files and classes on the command line to determine what annotations are present, the compiler queries the pro- cessors to determine what annotations they process. When a match is found, the processor will be invoked. A processor may "claim" the anno- tations it processes, in which case no further attempt is made to find any processors for those annotations. Once all annotations have been claimed, the compiler does not look for additional processors. If any processors generate any new source files, another round of anno- tation processing will occur: any newly generated source files will be scanned, and the annotations processed as before. Any processors invoked on previous rounds will also be invoked on all subsequent rounds. This continues until no new source files are generated. After a round occurs where no new source files are generated, the anno- tation processors will be invoked one last time, to give them a chance to complete any work they may need to do. Finally, unless the -proc:only option is used, the compiler will compile the original and all the generated source files. Implicitly loaded source files To compile a set of source files, the compiler may need to implicitly load additional source files. (See Searching For Types). Such files are currently not subject to annotation processing. By default, the com- piler will give a warning if annotation processing has occurred and any implicitly loaded source files are compiled. See the -implicit option for ways to suppress the warning. SEARCHING FOR TYPES When compiling a source file, the compiler often needs information about a type whose definition did not appear in the source files given on the command line. The compiler needs type information for every class or interface used, extended, or implemented in the source file. This includes classes and interfaces not explicitly mentioned in the source file but which provide information through inheritance. For example, when you subclass java.applet.Applet, you are also using Applet's ancestor classes: java.awt.Panel, java.awt.Container, java.awt.Component, and java.lang.Object. When the compiler needs type information, it looks for a source file or class file which defines the type. The compiler searches for class files first in the bootstrap and extension classes, then in the user class path (which by default is the current directory). The user class path is defined by setting the CLASSPATH environment variable or by using the -classpath command line option. (For details, see Setting the Class Path). If you set the -sourcepath option, the compiler searches the indicated path for source files; otherwise the compiler searches the user class path for both class files and source files. You can specify different bootstrap or extension classes with the -bootclasspath and -extdirs options; see Cross-Compilation Options below. A successful type search may produce a class file, a source file, or both. If both are found, you can use the -Xprefer option to instruct the compiler which to use. If newer is given, the compiler will use the newer of the two files. If source is given, it will use the source file. The default is newer. If a type search finds a source file for a required type, either by itself, or as a result of the setting for -Xprefer, the compiler will read the source file to get the information it needs. In addition, it will by default compile the source file as well. You can use the -implicit option to specify the behavior. If none is given, no class files will be generated for the source file. If class is given, class files will be generated for the source file. The compiler may not discover the need for some type information until after annotation processing is complete. If the type information is found in a source file and no -implicit option is given, the compiler will give a warning that the file is being compiled without being sub- ject to annotation processing. To disable the warning, either specify the file on the command line (so that it will be subject to annotation processing) or use the -implicit option to specify whether or not class files should be generated for such source files. PROGRAMMATIC INTERFACE javac supports the new Java Compiler API defined by the classes and interfaces in the javax.tools package. Example To perform a compilation using arguments as you would give on the com- mand line, you can use the following: JavaCompiler javac = ToolProvider.getSystemJavaCompiler(); int rc = javac.run(null, null, null, args); This will write any diagnostics to the standard output stream, and return the exit code that javac would give when invoked from the com- mand line. You can use other methods on the javax.tools.JavaCompiler interface to handle diagnostics, control where files are read from and written to, and so on. Old Interface Note: This API is retained for backwards compatibility only; all new code should use the Java Compiler API, described above. The com.sun.tools.javac.Main class provides two static methods to invoke the compiler from a program: public static int compile(String[] args); public static int compile(String[] args, PrintWriter out); The args parameter represents any of the command line arguments that would normally be passed to the javac program and are outlined in the above Synopsis section. The out parameter indicates where the compiler's diagnostic output is directed. The return value is equivalent to the exit value from javac. Note that all other classes and methods found in a package whose name starts with com.sun.tools.javac (informally known as sub-packages of com.sun.tools.javac) are strictly internal and subject to change at any time. EXAMPLES Compiling a Simple Program One source file, Hello.java, defines a class called greetings.Hello. The greetings directory is the package directory both for the source file and the class file and is off the current directory. This allows us to use the default user class path. It also makes it unnecessary to specify a separate destination directory with -d. % ls greetings/ % ls greetings Hello.java % cat greetings/Hello.java package greetings; public class Hello { public static void main(String[] args) { for (int i=0; i < args.length; i++) { System.out.println("Hello " + args[i]); } } } % javac greetings/Hello.java % ls greetings Hello.class Hello.java % java greetings.Hello World Universe Everyone Hello World Hello Universe Hello Everyone Compiling Multiple Source Files This example compiles all the source files in the package greetings. % ls greetings/ % ls greetings Aloha.java GutenTag.java Hello.java Hi.java % javac greetings/*.java % ls greetings Aloha.class GutenTag.class Hello.class Hi.class Aloha.java GutenTag.java Hello.java Hi.java Specifying a User Class Path Having changed one of the source files in the previous example, we recompile it: % pwd /examples % javac greetings/Hi.java Since greetings.Hi refers to other classes in the greetings package, the compiler needs to find these other classes. The example above works, because our default user class path happens to be the directory containing the package directory. But suppose we want to recompile this file and not worry about which directory we're in? Then we need to add /examples to the user class path. We can do this by setting CLASSPATH, but here we'll use the -classpath option. % javac -classpath /examples /examples/greetings/Hi.java If we change greetings.Hi again, to use a banner utility, that utility also needs to be accessible through the user class path. % javac -classpath /examples:/lib/Banners.jar /examples/greetings/Hi.java To execute a class in greetings, we need access both to greetings and to the classes it uses. % java -classpath /examples:/lib/Banners.jar greetings.Hi Separating Source Files and Class Files It often makes sense to keep source files and class files in separate directories, especially on large projects. We use -d to indicate the separate class file destination. Since the source files are not in the user class path, we use -sourcepath to help the compiler find them. % ls classes/ lib/ src/ % ls src farewells/ % ls src/farewells Base.java GoodBye.java % ls lib Banners.jar % ls classes % javac -sourcepath src -classpath classes:lib/Banners.jar src/farewells/GoodBye.java -d classes % ls classes farewells/ % ls classes/farewells Base.class GoodBye.class Note: The compiler compiled src/farewells/Base.java, even though we didn't specify it on the command line. To trace automatic compiles, use the -verbose option. Cross-Compilation Example The following example uses javac to compile code that will run on a 1.6 VM. % javac -source 1.6 -target 1.6 -bootclasspath jdk1.6.0/lib/rt.jar -extdirs "" OldCode.java The -source 1.6 option specifies that version 1.6 (or 6) of the Java programming language be used to compile OldCode.java. The option -tar- get 1.6 option ensures that the generated class files will be compati- ble with 1.6 VMs. Note that in most cases, the value of the -target option is the value of the -source option; in this example, you can omit the -target option. You must specify the -bootclasspath option to specify the correct ver- sion of the bootstrap classes (the rt.jar library). If not, the com- piler generates a warning: % javac -source 1.6 OldCode.java warning: [options] bootstrap class path not set in conjunction with -source 1.6 If you do not specify the correct version of bootstrap classes, the compiler will use the old language rules (in this example, it will use version 1.6 of the Java programming language) combined with the new bootstrap classes, which can result in class files that do not work on the older platform (in this case, Java SE 6) because reference to non-existent methods can get included. SEE ALSO o The javac Guide @ http://docs.oracle.com/javase/7/docs/tech- notes/guides/javac/index.html o java(1) - the Java Application Launcher o jdb(1) - Java Application Debugger o javah(1) - C Header and Stub File Generator o javap(1) - Class File Disassembler o javadoc(1) - API Documentation Generator o jar(1) - JAR Archive Tool o The Java Extensions Framework @ http://docs.oracle.com/javase/7/docs/technotes/guides/exten- sions/index.html
还是man厉害，不仅给出解释，还给出使用事例。