Hello World

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public class HelloWorld {
   public static void main(String[] args) {
     System.out.println("hello world");
   }
}

invokevirtual
$ javap -c HelloWorld

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Compiled from "HelloWorld.java"
public class HelloWorld {
  public HelloWorld();
    Code:
       0: aload_0
       // Method java/lang/Object."<init>":()V
       1: invokespecial #1
       4: return

  public static void main(java.lang.String[]);
    Code:
       // Field java/lang/System.out:Ljava/io/PrintStream;
       0: getstatic     #2
       // String hello world
       3: ldc           #3
       // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       5: invokevirtual #4
       8: return
}

The property out in System is a packaged type PrintStream.
There is a BufferedOutputStream inside PrintStream.

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public final class System {
    ...
    public final static PrintStream out = null;
    ...
    private static native void setOut0(PrintStream out);
    ...
    private static void initializeSystemClass() {
      ...
      FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
      ...
      setOut0(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));
      ...
    }
}

The init method called after thread initialization, initialize the system class.

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private static PrintStream newPrintStream(FileOutputStream fos, String enc) {
   if (enc != null) {
        try {
            return new PrintStream(new BufferedOutputStream(fos, 128), true, enc);
        } catch (UnsupportedEncodingException uee) {}
    }
    return new PrintStream(new BufferedOutputStream(fos, 128), true);
}

package buffer function into out. base on encoding, create output stream object.


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JNIEXPORT void JNICALL
Java_java_lang_System_setOut0(JNIEnv *env, jclass cla, jobject stream)
{
    jfieldID fid =
        (*env)->GetStaticFieldID(env,cla,"out","Ljava/io/PrintStream;");
    if (fid == 0)
        return;
    (*env)->SetStaticObjectField(env,cla,fid,stream);
}

  • The native method setOut0 will call this c function.

  • fid the member of System the out‘s id.

  • "Ljava/io/PrintStream;" mean java.io.PrintStream object

SetStaticObjectField define here.

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void (JNICALL *SetStaticObjectField)
      (JNIEnv *env, jclass clazz, jfieldID fieldID, jobject value);