How does javac automatically compile dependencies of a class

问题

Given the following directory structure:

/top
   |--- wrk
          |--- pkg
                  |--- A.java
                  |--- B.java

Assume that the two files A.java and B.java contain the following code, respectively:

// Filename: A.java
package pkg;
class A { B b; }

// Filename: B.java
package pkg;
class B {...}

Assuming that the current directory is /top/wrk

Why does the command javac -cp . pkg/A.java work successfully even though we have not yet compiled B.java?

Also if the current directory is /top/wrk/pkg then the command javac A.java works. How so?

回答1:

Why does the command javac -cp . pkg/A.java work successfully even though we have not yet compiled B.java

When you compile A.java, the compiler will compile B.java as well since both A.java and B.java are in the same package. This will work even If B.java was in a different package from A.java (provided B is public) as long as both the packages are present in the wrk directory and you compile A.java from wrk directory.

From the Oracle documentation for javac :

If the -sourcepath option is not specified, the user class path is also searched for source files.

From the Oracle document for CLASSPATH

The default value of the class path is "."

If you haven't set a CLASSPATH, it will be defaulted to .. Subsequently, the sourcepath will also be . since the default sourcepath is the same as the CLASSPATH. You can confirm that the default sourcepath is set to . by compiling A.java using javac -verbose -g pkg\A.java. Notice that the compiler is looking in the current directory for .java files :

[parsing started pkg\A.java] [parsing completed 29ms] [search path for source files: [.]]

To confirm that the sourcepath is set to CLASSPATH, you can try changing the CLASSPATH using the -cp option by compiling A.java using javac -cp C:\ -verbose -g pkg\A.java. A.java will not compile this time since you have overwritten the CLASSPATH to C:\ and that's what sourcepath will default to as well. This is the output :

[parsing started pkg\A.java] [parsing completed 26ms] [search path for source files: [C:\]] pkg\A.java:3: cannot find symbol symbol : class B

Also if the current directory is /top/wrk/pkg then the command javac A.java works. How so?

This will not work regardless of whether B.class is present in pkg

Disclaimer : I can only confirm this behavior on Windows but I highly doubt that it should be any different on other operating systems.

回答2:

The compiler has to either find and successfully compile a source for B, or find a .class for B even if it's just an import. As opposed to loading, which is done dynamically.

Look in your output directory and you'll see that B has been compiled as well. It will be compiled even if in a different package, but you'll have to make it public to reference it from A.

回答3:

From the Oracle javac docs...

If you set the -sourcepath option, then the compiler searches the indicated path for source files. Otherwise, the compiler searches the user class path for both class files and source files. On Windows, the -sourcepath option seems to be set by default and your command works.

On my Mac though, it fails and gives following message...

A.java:5: error: cannot find symbol
    B b;
    ^
  symbol:   class B
  location: class A
1 error

To get it to automatically look and compile source files of dependencies, you would need to use the -sourcepath option. e.g...

javac -sourcepath ./* A.java

来源：https://stackoverflow.com/questions/30527632/how-does-javac-automatically-compile-dependencies-of-a-class

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