提升JAVA程序的性能

清单 1：描述由字符串连接符‘+’所产生的字节代码操作的伪代码

create new String (STR_1)

duplicate the String

load the constant String "Joe" (STR_2)

call String constructor

store this String in the local variable array position 0

get the static out field from the java.io.PrintStream class (OUT)

create a new StringBuffer (STR_BUF_1)

duplicate the StringBuffer

call StringBuffer constructor

store this StringBuffer in the local variable array position 1

invoke the append method on STR_BUF_1 with STR_1 as the argument

load the constant String " is my name." (STR_3)

invoke the append method on STR_BUF_1 with STR_3 as the argument

invoke toString method on STR_BUF_1 (STR_4)

invoke the println method on OUT

清单2：使用StringBuffer的append操作符的字节代码/伪代码

create new StringBuffer (STR_BUF_1)

duplicate the StringBuffer

load the constant String "Joe" (STR_1)

call StringBuffer constructor

store this StringBuffer in the local variable array position 1

get the static out field from the java.io.PrintStream class (OUT)

load STR_BUF_1

load the constant String " is my name." (STR_2)

invoke the append method on STR_BUF_1 with STR_2 as the argument

invoke toString method on STR_BUF_1 (STR_3)

invoke the println method on OUT

清单3：可以进行配置使其在发布版本中不产生代码的]单元素的Logger类

public class Logger {

  // 这个类的实例

  private static Logger theLogger = null;

  // 调试消息（debug messages）的控制器，设为true时允许调试消息，false则反之

  public static final boolean CAN_DEBUG = true;

  /** 私有的构造函数——只允许一个实例 */

  private Logger() {}

  /** 返回这个类所创建的唯一实例 */

  public static Logger getInstance() {

    if(theLogger == null) { theLogger = new Logger(); }

    return theLogger;

  }

  public void debugMsg(String msg) { System.out.println(msg); }

}

清单4：提供可配置日志记录级别Logger类

public class Logger {

  ...

  // 信息消息的控制器

  public static final int CAN_INFO = 1;

  // 调试消息的控制器

  public static final int CAN_DEBUG = 2;

  // 调试级别——缺省为信息消息

  public int LOG_LEVEL = 1;

  ...

  public void setLogLevel(int level) {

    if(level >= 0) { LOG_LEVEL = level; }

  }

  /** 如果CAN_INFO位被设置则返回true */

  public boolean canInfo() {

    if((LOG_LEVEL & CAN_INFO) == CAN_INFO) { return true; }

    return false;

  }

  /** 如果CAN_DEBUG位被设置则返回true */

  public boolean canDebug() {

    if((LOG_LEVEL & CAN_DEBUG) == CAN_DEBUG) { return true; }

    return false;

  }

  public void debugMsg(String msg) { System.out.println(msg); }

  public void infoMsg(String msg) { System.out.println(msg); }

}

清单5：TestLogger.java—定义简单API的接口的示例实现

/* Copyright (c) 2000 Stepping Stone Software Ltd, John Keyes */

public class TestLogger implements LogAPI {

  static Logger myLogger = Logger.getInstance();

  StringBuffer msg = new StringBuffer();

  public static void main(String args[]) {

    String strLevel = System.getProperty("app.loglevel");

    if(strLevel != null) {

    int level = Integer.parseInt(strLevel);

    myLogger.setLogLevel(level);

    }

    TestLogger testLog = new TestLogger();

    testLog.test();

  }

  TestLogger() { }

  public void test() {

    int age = 24;

    String name = "Joe";

    if(myLogger.canDebug()) {

      createMsg();

      appendLog(" DEBUG/n Name:");

      appendLog(name);

      appendLog(" Age:");

      appendLog(age);

      logDebugMsg();

    }

    if(myLogger.canInfo()) {

      createMsg();

      appendLog(" INFO/n Name:");

      appendLog(name);

      appendLog(" Age:");

      appendLog(age);

      logInfoMsg();

    }

  }

  public void createMsg() { msg.setLength(0); }

  public void appendLog(String str) { msg.append(str); }

  public void appendLog(int i) { msg.append(i); }

  public void logDebugMsg() {

    myLogger.debugMsg(msg.toString());

  }

  public void logInfoMsg() {

    myLogger.infoMsg(msg.toString());

  }

}

清单6：StringVector.java——String类型集合的示例实现

/* Copyright (c) 2000 Stepping Stone Software Ltd, John Keyes */

public class StringVector {

  private String [] data;

  private int count;

  public StringVector() { this(10); // 缺省尺寸为10 }

  public StringVector(int initialSize) {

    data = new String[initialSize];

  }

  public void add(String str) {

    // 忽略null字符串

    if(str == null) { return; }

    ensureCapacity(count + 1);

    data[count++] = str;

  }

  private void ensureCapacity(int minCapacity) {

    int oldCapacity = data.length;

    if (minCapacity > oldCapacity) {

      String oldData[] = data;

      int newCapacity = oldCapacity * 2;

      data = new String[newCapacity];

      System.arraycopy(oldData, 0, data, 0, count);

    }

  }

  public void remove(String str) {

    if(str == null) { return; //忽略null字符串}

    for(int i = 0; i < count; i++) {      

      // 查找匹配

      if(data[i].equals(str)) {

    System.arraycopy(data,i+1,data,i,count-1); //复制数据

         data[--count] = null;

    return;

      }

    }

  }

  public final String getStringAt(int index) {

    if(index < 0) { return null; }     

    else if(index > count) {

       return null; // index is > # strings

    }

    else { return data[index]; // index is good }

  }

  // not shown: size(), toStringArray()

}

清单7：TestCollection.java——显著地体现了性能的差异

/* Copyright (c) 2000 Stepping Stone Software Ltd, John Keyes */

import java.util.Vector;

public class TestCollection {

   public static void main(String args []) {

      TestCollection collect = new TestCollection();

      if(args.length == 0) {

         System.out.println(

         "Usage: java TestCollection [ vector | stringvector ]");

         System.exit(1);

      }

      if(args[0].equals("vector")) {

         Vector store = new Vector();

         long start = System.currentTimeMillis();

         for(int i = 0; i < 1000000; i++) {

            store.addElement("string");

         }

         long finish = System.currentTimeMillis();

         System.out.println((finish-start));

         start = System.currentTimeMillis();

         for(int i = 0; i < 1000000; i++) {

            String result = (String)store.elementAt(i);

         }

         finish = System.currentTimeMillis();

         System.out.println((finish-start));

      }

      else if(args[0].equals("stringvector")) {

         StringVector store = new StringVector();

         long start = System.currentTimeMillis();

         for(int i = 0; i < 1000000; i++) { store.add("string"); }

         long finish = System.currentTimeMillis();

         System.out.println((finish-start));

         start = System.currentTimeMillis();

         for(int i = 0; i < 1000000; i++) {

            String result = store.getStringAt(i);

         }

         finish = System.currentTimeMillis();

         System.out.println((finish-start));

      }

   }

}