首先自定义一个OutputStream

import java.io.OutputStream;
import javax.swing.JTextArea;

/**
 *
 * @author 杨庆成 */
public class GuiOutputStream extends OutputStream{
    private JTextArea _text;
    private byte _c;
    private boolean _b=false;
   
    /** Creates a new instance of GuiOutputStream */
    public GuiOutputStream(JTextArea text) {
        _text=text;
    }
   
    public void write(int i){
        try{
            byte c=(byte)i;
            if(_b){
                _b=false;
                byte[] bs=new byte[2];
                bs[0]=_c;
                bs[1]=c;
                _text.append(new String(bs));
            }
            else if(c>0){
                _b=false;
                _text.append(String.valueOf((char)c));
            }
            else{
                _b=true;
                _c=c;
            }
        }
        catch(Exception ex){
            ex.printStackTrace();
        }
    }
   
}

使用方式

重新定向OutputStream
        JTextArea jTextArea1 = new javax.swing.JTextArea();

        PrintStream out=new PrintStream(new GuiOutputStream(jTextArea1));
        System.setOut(out);

Log4j配置文件中Appender设置成org.apache.log4j.ConsoleAppender