Java编程最差实践
原文地址:http://www.odi.ch/prog/design/newbies.php
每天在写Java程序, 其实里面有一些细节大家可能没怎么注意, 这不, 有人总结了一个我们编程中常见的问题. 虽然一般没有什么大问题, 但是最好别这样做. 另外这里提到的很多问题其实可以通过Findbugs( http://findbugs.sourceforge.net/ )来帮我们进行检查出来.
字符串连接误用 
错误的写法:
- String s = "";
- for (Person p : persons) {
- s += ", " + p.getName();
- }
- s = s.substring(2); //remove first comma
String s = "";
for (Person p : persons) {
s += ", " + p.getName();
}
s = s.substring(2); //remove first comma
正确的写法:
- StringBuilder sb = new StringBuilder(persons.size() * 16); // well estimated buffer
- for (Person p : persons) {
- if (sb.length() > 0) sb.append(", ");
- sb.append(p.getName);
- }
StringBuilder sb = new StringBuilder(persons.size() * 16); // well estimated buffer
for (Person p : persons) {
if (sb.length() > 0) sb.append(", ");
sb.append(p.getName);
}
错误的使用StringBuffer 错误的写法:
- StringBuffer sb = new StringBuffer();
- sb.append("Name: ");
- sb.append(name + '\n');
- sb.append("!");
- ...
- String s = sb.toString();
StringBuffer sb = new StringBuffer();
sb.append("Name: ");
sb.append(name + '\n');
sb.append("!");
...
String s = sb.toString();
问题在第三行, append char比String性能要好, 另外就是初始化StringBuffer没有指定size, 导致中间append时可能重新调整内部数组大小. 如果是JDK1.5最好用StringBuilder取代StringBuffer, 除非有线程安全的要求. 还有一种方式就是可以直接连接字符串. 缺点就是无法初始化时指定长度. 正确的写法:
- StringBuilder sb = new StringBuilder(100);
- sb.append("Name: ");
- sb.append(name);
- sb.append("\n!");
- String s = sb.toString();
StringBuilder sb = new StringBuilder(100);
sb.append("Name: ");
sb.append(name);
sb.append("\n!");
String s = sb.toString();
或者这样写:
- String s = "Name: " + name + "\n!";
String s = "Name: " + name + "\n!";
测试字符串相等性 错误的写法:
- if (name.compareTo("John") == 0) ...
- if (name == "John") ...
- if (name.equals("John")) ...
- if ("".equals(name)) ...
if (name.compareTo("John") == 0) ...
if (name == "John") ...
if (name.equals("John")) ...
if ("".equals(name)) ...
上面的代码没有错, 但是不够好. compareTo不够简洁, ==原义是比较两个对象是否一样. 另外比较字符是否为空, 最好判断它的长度. 正确的写法:
- if ("John".equals(name)) ...
- if (name.length() == 0) ...
- if (name.isEmpty()) ...
if ("John".equals(name)) ...
if (name.length() == 0) ...
if (name.isEmpty()) ...
数字转换成字符串 错误的写法:
- "" + set.size()
- new Integer(set.size()).toString()
"" + set.size()
new Integer(set.size()).toString() 正确的写法:
- String.valueOf(set.size())
String.valueOf(set.size())
利用不可变对象(Immutable) 错误的写法:
- zero = new Integer(0);
- return Boolean.valueOf("true");
zero = new Integer(0);
return Boolean.valueOf("true"); 正确的写法:
- zero = Integer.valueOf(0);
- return Boolean.TRUE;
zero = Integer.valueOf(0);
return Boolean.TRUE;
请使用XML解析器 错误的写法:
- int start = xml.indexOf("
" ) + "" .length(); - int end = xml.indexOf("");
- String name = xml.substring(start, end);
int start = xml.indexOf("") + "".length();
int end = xml.indexOf(" ");
String name = xml.substring(start, end); 正确的写法:
- SAXBuilder builder = new SAXBuilder(false);
- Document doc = doc = builder.build(new StringReader(xml));
- String name = doc.getRootElement().getChild("name").getText();
SAXBuilder builder = new SAXBuilder(false);
Document doc = doc = builder.build(new StringReader(xml));
String name = doc.getRootElement().getChild("name").getText();
请使用JDom组装XML 错误的写法:
- String name = ...
- String attribute = ...
- String xml = "
" - +"
- +"";
String name = ...
String attribute = ...
String xml = ""
+""+ name +" "
+" "; 正确的写法:
- Element root = new Element("root");
- root.setAttribute("att", attribute);
- root.setText(name);
- Document doc = new Documet();
- doc.setRootElement(root);
- XmlOutputter out = new XmlOutputter(Format.getPrettyFormat());
- String xml = out.outputString(root);
Element root = new Element("root");
root.setAttribute("att", attribute);
root.setText(name);
Document doc = new Documet();
doc.setRootElement(root);
XmlOutputter out = new XmlOutputter(Format.getPrettyFormat());
String xml = out.outputString(root);
XML编码陷阱 错误的写法:
- String xml = FileUtils.readTextFile("my.xml");
String xml = FileUtils.readTextFile("my.xml");
因为xml的编码在文件中指定的, 而在读文件的时候必须指定编码. 另外一个问题不能一次就将一个xml文件用String保存, 这样对内存会造成不必要的浪费, 正确的做法用InputStream来边读取边处理. 为了解决编码的问题, 最好使用XML解析器来处理
未指定字符编码 错误的写法:
- Reader r = new FileReader(file);
- Writer w = new FileWriter(file);
- Reader r = new InputStreamReader(inputStream);
- Writer w = new OutputStreamWriter(outputStream);
- String s = new String(byteArray); // byteArray is a byte[]
- byte[] a = string.getBytes();
Reader r = new FileReader(file);
Writer w = new FileWriter(file);
Reader r = new InputStreamReader(inputStream);
Writer w = new OutputStreamWriter(outputStream);
String s = new String(byteArray); // byteArray is a byte[]
byte[] a = string.getBytes();
这样的代码主要不具有跨平台可移植性. 因为不同的平台可能使用的是不同的默认字符编码. 正确的写法:
- Reader r = new InputStreamReader(new FileInputStream(file), "ISO-8859-1");
- Writer w = new OutputStreamWriter(new FileOutputStream(file), "ISO-8859-1");
- Reader r = new InputStreamReader(inputStream, "UTF-8");
- Writer w = new OutputStreamWriter(outputStream, "UTF-8");
- String s = new String(byteArray, "ASCII");
- byte[] a = string.getBytes("ASCII");
Reader r = new InputStreamReader(new FileInputStream(file), "ISO-8859-1");
Writer w = new OutputStreamWriter(new FileOutputStream(file), "ISO-8859-1");
Reader r = new InputStreamReader(inputStream, "UTF-8");
Writer w = new OutputStreamWriter(outputStream, "UTF-8");
String s = new String(byteArray, "ASCII");
byte[] a = string.getBytes("ASCII");
未对数据流进行缓存 错误的写法:
- InputStream in = new FileInputStream(file);
- int b;
- while ((b = in.read()) != -1) {
- ...
- }
InputStream in = new FileInputStream(file);
int b;
while ((b = in.read()) != -1) {
...
}
上面的代码是一个byte一个byte的读取, 导致频繁的本地JNI文件系统访问, 非常低效, 因为调用本地方法是非常耗时的. 最好用BufferedInputStream包装一下. 曾经做过一个测试, 从/dev/zero下读取1MB, 大概花了1s, 而用BufferedInputStream包装之后只需要60ms, 性能提高了94%! 这个也适用于output stream操作以及socket操作. 正确的写法:
- InputStream in = new BufferedInputStream(new FileInputStream(file));
InputStream in = new BufferedInputStream(new FileInputStream(file));
无限使用heap内存 错误的写法:
- byte[] pdf = toPdf(file);
byte[] pdf = toPdf(file);
这里有一个前提, 就是文件大小不能讲JVM的heap撑爆. 否则就等着OOM吧, 尤其是在高并发的服务器端代码. 最好的做法是采用Stream的方式边读取边存储(本地文件或database). 正确的写法:
- File pdf = toPdf(file);
File pdf = toPdf(file);
另外, 对于服务器端代码来说, 为了系统的安全, 至少需要对文件的大小进行限制.
不指定超时时间
错误的代码:
- Socket socket = ...
- socket.connect(remote);
- InputStream in = socket.getInputStream();
- int i = in.read();
Socket socket = ...
socket.connect(remote);
InputStream in = socket.getInputStream();
int i = in.read();
这种情况在工作中已经碰到不止一次了. 个人经验一般超时不要超过20s. 这里有一个问题, connect可以指定超时时间, 但是read无法指定超时时间. 但是可以设置阻塞(block)时间. 正确的写法:
- Socket socket = ...
- socket.connect(remote, 20000); // fail after 20s
- InputStream in = socket.getInputStream();
- socket.setSoTimeout(15000);
- int i = in.read();
Socket socket = ...
socket.connect(remote, 20000); // fail after 20s
InputStream in = socket.getInputStream();
socket.setSoTimeout(15000);
int i = in.read();
另外, 文件的读取(FileInputStream, FileChannel, FileDescriptor, File)没法指定超时时间, 而且IO操作均涉及到本地方法调用, 这个更操作了JVM的控制范围, 在分布式文件系统中, 对IO的操作内部实际上是网络调用. 一般情况下操作60s的操作都可以认为已经超时了. 为了解决这些问题, 一般采用缓存和异步/消息队列处理.
频繁使用计时器
错误代码:
- for (...) {
- long t = System.currentTimeMillis();
- long t = System.nanoTime();
- Date d = new Date();
- Calendar c = new GregorianCalendar();
- }
for (...) {
long t = System.currentTimeMillis();
long t = System.nanoTime();
Date d = new Date();
Calendar c = new GregorianCalendar();
}
每次new一个Date或Calendar都会涉及一次本地调用来获取当前时间(尽管这个本地调用相对其他本地方法调用要快).
如果对时间不是特别敏感, 这里使用了clone方法来新建一个Date实例. 这样相对直接new要高效一些. 正确的写法:
- Date d = new Date();
- for (E entity : entities) {
- entity.doSomething();
- entity.setUpdated((Date) d.clone());
- }
Date d = new Date();
for (E entity : entities) {
entity.doSomething();
entity.setUpdated((Date) d.clone());
}
如果循环操作耗时较长(超过几ms), 那么可以采用下面的方法, 立即创建一个Timer, 然后定期根据当前时间更新时间戳, 在我的系统上比直接new一个时间对象快200倍:
- private volatile long time;
- Timer timer = new Timer(true);
- try {
- time = System.currentTimeMillis();
- timer.scheduleAtFixedRate(new TimerTask() {
- public void run() {
- time = System.currentTimeMillis();
- }
- }, 0L, 10L); // granularity 10ms
- for (E entity : entities) {
- entity.doSomething();
- entity.setUpdated(new Date(time));
- }
- } finally {
- timer.cancel();
- }
private volatile long time;
Timer timer = new Timer(true);
try {
time = System.currentTimeMillis();
timer.scheduleAtFixedRate(new TimerTask() {
public void run() {
time = System.currentTimeMillis();
}
}, 0L, 10L); // granularity 10ms
for (E entity : entities) {
entity.doSomething();
entity.setUpdated(new Date(time));
}
} finally {
timer.cancel();
}
捕获所有的异常 错误的写法:
- Query q = ...
- Person p;
- try {
- p = (Person) q.getSingleResult();
- } catch(Exception e) {
- p = null;
- }
Query q = ...
Person p;
try {
p = (Person) q.getSingleResult();
} catch(Exception e) {
p = null;
}
这是EJB3的一个查询操作, 可能出现异常的原因是: 结果不唯一; 没有结果; 数据库无法访问, 而捕获所有的异常, 设置为null将掩盖各种异常情况. 正确的写法:
- Query q = ...
- Person p;
- try {
- p = (Person) q.getSingleResult();
- } catch(NoResultException e) {
- p = null;
- }
Query q = ...
Person p;
try {
p = (Person) q.getSingleResult();
} catch(NoResultException e) {
p = null;
}
忽略所有异常
- try {
- doStuff();
- } catch(Exception e) {
- log.fatal("Could not do stuff");
- }
- doMoreStuff();
try {
doStuff();
} catch(Exception e) {
log.fatal("Could not do stuff");
}
doMoreStuff();
这个代码有两个问题, 一个是没有告诉调用者, 系统调用出错了. 第二个是日志没有出错原因, 很难跟踪定位问题. 正确的写法:
- try {
- doStuff();
- } catch(Exception e) {
- throw new MyRuntimeException("Could not do stuff because: "+ e.getMessage, e);
- }
try {
doStuff();
} catch(Exception e) {
throw new MyRuntimeException("Could not do stuff because: "+ e.getMessage, e);
}
重复包装RuntimeException 错误的写法:
- try {
- doStuff();
- } catch(Exception e) {
- throw new RuntimeException(e);
- }
try {
doStuff();
} catch(Exception e) {
throw new RuntimeException(e);
} 正确的写法:
- try {
- doStuff();
- } catch(RuntimeException e) {
- throw e;
- } catch(Exception e) {
- throw new RuntimeException(e.getMessage(), e);
- }
- try {
- doStuff();
- } catch(IOException e) {
- throw new RuntimeException(e.getMessage(), e);
- } catch(NamingException e) {
- throw new RuntimeException(e.getMessage(), e);
- }
try {
doStuff();
} catch(RuntimeException e) {
throw e;
} catch(Exception e) {
throw new RuntimeException(e.getMessage(), e);
}
try {
doStuff();
} catch(IOException e) {
throw new RuntimeException(e.getMessage(), e);
} catch(NamingException e) {
throw new RuntimeException(e.getMessage(), e);
}
不正确的传播异常 错误的写法:
- try {
- } catch(ParseException e) {
- throw new RuntimeException();
- throw new RuntimeException(e.toString());
- throw new RuntimeException(e.getMessage());
- throw new RuntimeException(e);
- }
try {
} catch(ParseException e) {
throw new RuntimeException();
throw new RuntimeException(e.toString());
throw new RuntimeException(e.getMessage());
throw new RuntimeException(e);
}
主要是没有正确的将内部的错误信息传递给调用者. 第一个完全丢掉了内部错误信息, 第二个错误信息依赖toString方法, 如果没有包含最终的嵌套错误信息, 也会出现丢失, 而且可读性差. 第三个稍微好一些, 第四个跟第二个一样. 正确的写法:
- try {
- } catch(ParseException e) {
- throw new RuntimeException(e.getMessage(), e);
- }
try {
} catch(ParseException e) {
throw new RuntimeException(e.getMessage(), e);
}
用日志记录异常 错误的写法:
- try {
- ...
- } catch(ExceptionA e) {
- log.error(e.getMessage(), e);
- throw e;
- } catch(ExceptionB e) {
- log.error(e.getMessage(), e);
- throw e;
- }
try {
...
} catch(ExceptionA e) {
log.error(e.getMessage(), e);
throw e;
} catch(ExceptionB e) {
log.error(e.getMessage(), e);
throw e;
}
一般情况下在日志中记录异常是不必要的, 除非调用方没有记录日志.
异常处理不彻底 错误的写法:
- try {
- is = new FileInputStream(inFile);
- os = new FileOutputStream(outFile);
- } finally {
- try {
- is.close();
- os.close();
- } catch(IOException e) {
- /* we can't do anything */
- }
- }
try {
is = new FileInputStream(inFile);
os = new FileOutputStream(outFile);
} finally {
try {
is.close();
os.close();
} catch(IOException e) {
/* we can't do anything */
}
}
is可能close失败, 导致os没有close 正确的写法:
- try {
- is = new FileInputStream(inFile);
- os = new FileOutputStream(outFile);
- } finally {
- try { if (is != null) is.close(); } catch(IOException e) {/* we can't do anything */}
- try { if (os != null) os.close(); } catch(IOException e) {/* we can't do anything */}
- }
try {
is = new FileInputStream(inFile);
os = new FileOutputStream(outFile);
} finally {
try { if (is != null) is.close(); } catch(IOException e) {/* we can't do anything */}
try { if (os != null) os.close(); } catch(IOException e) {/* we can't do anything */}
}
捕获不可能出现的异常 错误的写法:
- try {
- ... do risky stuff ...
- } catch(SomeException e) {
- // never happens
- }
- ... do some more ...
try {
... do risky stuff ...
} catch(SomeException e) {
// never happens
}
... do some more ... 正确的写法:
- try {
- ... do risky stuff ...
- } catch(SomeException e) {
- // never happens hopefully
- throw new IllegalStateException(e.getMessage(), e); // crash early, passing all information
- }
- ... do some more ...
try {
... do risky stuff ...
} catch(SomeException e) {
// never happens hopefully
throw new IllegalStateException(e.getMessage(), e); // crash early, passing all information
}
... do some more ...
transient的误用 错误的写法:
- public class A implements Serializable {
- private String someState;
- private transient Log log = LogFactory.getLog(getClass());
- public void f() {
- log.debug("enter f");
- ...
- }
- }
public class A implements Serializable {
private String someState;
private transient Log log = LogFactory.getLog(getClass());
public void f() {
log.debug("enter f");
...
}
}
这里的本意是不希望Log对象被序列化. 不过这里在反序列化时, 会因为log未初始化, 导致f()方法抛空指针, 正确的做法是将log定义为静态变量或者定位为具备变量. 正确的写法:
- public class A implements Serializable {
- private String someState;
- private static final Log log = LogFactory.getLog(A.class);
- public void f() {
- log.debug("enter f");
- ...
- }
- }
- public class A implements Serializable {
- private String someState;
- public void f() {
- Log log = LogFactory.getLog(getClass());
- log.debug("enter f");
- ...
- }
- }
public class A implements Serializable {
private String someState;
private static final Log log = LogFactory.getLog(A.class);
public void f() {
log.debug("enter f");
...
}
}
public class A implements Serializable {
private String someState;
public void f() {
Log log = LogFactory.getLog(getClass());
log.debug("enter f");
...
}
}
不必要的初始化 错误的写法:
- public class B {
- private int count = 0;
- private String name = null;
- private boolean important = false;
- }
public class B {
private int count = 0;
private String name = null;
private boolean important = false;
}
这里的变量会在初始化时使用默认值:0, null, false, 因此上面的写法有些多此一举. 正确的写法:
- public class B {
- private int count;
- private String name;
- private boolean important;
- }
public class B {
private int count;
private String name;
private boolean important;
}
最好用静态final定义Log变量
- private static final Log log = LogFactory.getLog(MyClass.class);
private static final Log log = LogFactory.getLog(MyClass.class);
这样做的好处有三:
- 可以保证线程安全
- 静态或非静态代码都可用
- 不会影响对象序列化
选择错误的类加载器
错误的代码:
- Class clazz = Class.forName(name);
- Class clazz = getClass().getClassLoader().loadClass(name);
Class clazz = Class.forName(name);
Class clazz = getClass().getClassLoader().loadClass(name);
这里本意是希望用当前类来加载希望的对象, 但是这里的getClass()可能抛出异常, 特别在一些受管理的环境中, 比如应用服务器, web容器, Java WebStart环境中, 最好的做法是使用当前应用上下文的类加载器来加载. 正确的写法:
- ClassLoader cl = Thread.currentThread().getContextClassLoader();
- if (cl == null) cl = MyClass.class.getClassLoader(); // fallback
- Class clazz = cl.loadClass(name);
ClassLoader cl = Thread.currentThread().getContextClassLoader();
if (cl == null) cl = MyClass.class.getClassLoader(); // fallback
Class clazz = cl.loadClass(name);
反射使用不当 错误的写法:
- Class beanClass = ...
- if (beanClass.newInstance() instanceof TestBean) ...
Class beanClass = ...
if (beanClass.newInstance() instanceof TestBean) ...
这里的本意是检查beanClass是否是TestBean或是其子类, 但是创建一个类实例可能没那么简单, 首先实例化一个对象会带来一定的消耗, 另外有可能类没有定义默认构造函数. 正确的做法是用Class.isAssignableFrom(Class) 方法. 正确的写法:
- Class beanClass = ...
- if (TestBean.class.isAssignableFrom(beanClass)) ...
Class beanClass = ...
if (TestBean.class.isAssignableFrom(beanClass)) ...
不必要的同步 错误的写法:
- Collection l = new Vector();
- for (...) {
- l.add(object);
- }
Collection l = new Vector();
for (...) {
l.add(object);
}
Vector是ArrayList同步版本. 正确的写法:
- Collection l = new ArrayList();
- for (...) {
- l.add(object);
- }
Collection l = new ArrayList();
for (...) {
l.add(object);
}
错误的选择List类型
根据下面的表格数据来进行选择
| ArrayList | LinkedList | |
| add (append) | O(1) or ~O(log(n)) if growing | O(1) |
| insert (middle) | O(n) or ~O(n*log(n)) if growing | O(n) |
| remove (middle) | O(n) (always performs complete copy) | O(n) |
| iterate | O(n) | O(n) |
| get by index | O(1) | O(n) |
HashMap size陷阱 错误的写法:
- Map map = new HashMap(collection.size());
- for (Object o : collection) {
- map.put(o.key, o.value);
- }
Map map = new HashMap(collection.size());
for (Object o : collection) {
map.put(o.key, o.value);
}
这里可以参考guava的Maps.newHashMapWithExpectedSize的实现. 用户的本意是希望给HashMap设置初始值, 避免扩容(resize)的开销. 但是没有考虑当添加的元素数量达到HashMap容量的75%时将出现resize. 正确的写法:
- Map map = new HashMap(1 + (int) (collection.size() / 0.75));
Map map = new HashMap(1 + (int) (collection.size() / 0.75));
对Hashtable, HashMap 和 HashSet了解不够
这里主要需要了解HashMap和Hashtable的内部实现上, 它们都使用Entry包装来封装key/value, Entry内部除了要保存Key/Value的引用, 还需要保存hash桶中next Entry的应用, 因此对内存会有不小的开销, 而HashSet内部实现其实就是一个HashMap. 有时候IdentityHashMap可以作为一个不错的替代方案. 它在内存使用上更有效(没有用Entry封装, 内部采用Object[]). 不过需要小心使用. 它的实现违背了Map接口的定义. 有时候也可以用ArrayList来替换HashSet.
这一切的根源都是由于JDK内部没有提供一套高效的Map和Set实现.
对List的误用
建议下列场景用Array来替代List:
- list长度固定, 比如一周中的每一天
- 对list频繁的遍历, 比如超过1w次
- 需要对数字进行包装(主要JDK没有提供基本类型的List)
比如下面的代码. 错误的写法:
- List
codes = new ArrayList(); - codes.add(Integer.valueOf(10));
- codes.add(Integer.valueOf(20));
- codes.add(Integer.valueOf(30));
- codes.add(Integer.valueOf(40));
List codes = new ArrayList();
codes.add(Integer.valueOf(10));
codes.add(Integer.valueOf(20));
codes.add(Integer.valueOf(30));
codes.add(Integer.valueOf(40)); 正确的写法:
- int[] codes = { 10, 20, 30, 40 };
int[] codes = { 10, 20, 30, 40 }; 错误的写法:
- // horribly slow and a memory waster if l has a few thousand elements (try it yourself!)
- List
l = ...; - for (int i=0; i < l.size()-1; i++) {
- Mergeable one = l.get(i);
- Iterator
j = l.iterator(i+ 1); // memory allocation! - while (j.hasNext()) {
- Mergeable other = l.next();
- if (one.canMergeWith(other)) {
- one.merge(other);
- other.remove();
- }
- }
- }
// horribly slow and a memory waster if l has a few thousand elements (try it yourself!)
List l = ...;
for (int i=0; i < l.size()-1; i++) {
Mergeable one = l.get(i);
Iterator j = l.iterator(i+1); // memory allocation!
while (j.hasNext()) {
Mergeable other = l.next();
if (one.canMergeWith(other)) {
one.merge(other);
other.remove();
}
}
} 正确的写法:
- // quite fast and no memory allocation
- Mergeable[] l = ...;
- for (int i=0; i < l.length-1; i++) {
- Mergeable one = l[i];
- for (int j=i+1; j < l.length; j++) {
- Mergeable other = l[j];
- if (one.canMergeWith(other)) {
- one.merge(other);
- l[j] = null;
- }
- }
- }
// quite fast and no memory allocation
Mergeable[] l = ...;
for (int i=0; i < l.length-1; i++) {
Mergeable one = l[i];
for (int j=i+1; j < l.length; j++) {
Mergeable other = l[j];
if (one.canMergeWith(other)) {
one.merge(other);
l[j] = null;
}
}
}
实际上Sun也意识到这一点, 因此在JDK中, Collections.sort()就是将一个List拷贝到一个数组中然后调用Arrays.sort方法来执行排序.
用数组来描述一个结构
错误用法:
- /**
- * @returns [1]: Location, [2]: Customer, [3]: Incident
- */
- Object[] getDetails(int id) {...
/**
* @returns [1]: Location, [2]: Customer, [3]: Incident
*/
Object[] getDetails(int id) {...
这里用数组+文档的方式来描述一个方法的返回值. 虽然很简单, 但是很容易误用, 正确的做法应该是定义个类. 正确的写法:
- Details getDetails(int id) {...}
- private class Details {
- public Location location;
- public Customer customer;
- public Incident incident;
- }
Details getDetails(int id) {...}
private class Details {
public Location location;
public Customer customer;
public Incident incident;
}
对方法过度限制
错误用法:
- public void notify(Person p) {
- ...
- sendMail(p.getName(), p.getFirstName(), p.getEmail());
- ...
- }
- class PhoneBook {
- String lookup(String employeeId) {
- Employee emp = ...
- return emp.getPhone();
- }
- }
public void notify(Person p) {
...
sendMail(p.getName(), p.getFirstName(), p.getEmail());
...
}
class PhoneBook {
String lookup(String employeeId) {
Employee emp = ...
return emp.getPhone();
}
}
第一个例子是对方法参数做了过多的限制, 第二个例子对方法的返回值做了太多的限制. 正确的写法:
- public void notify(Person p) {
- ...
- sendMail(p);
- ...
- }
- class EmployeeDirectory {
- Employee lookup(String employeeId) {
- Employee emp = ...
- return emp;
- }
- }
public void notify(Person p) {
...
sendMail(p);
...
}
class EmployeeDirectory {
Employee lookup(String employeeId) {
Employee emp = ...
return emp;
}
}
对POJO的setter方法画蛇添足 错误的写法:
- private String name;
- public void setName(String name) {
- this.name = name.trim();
- }
- public void String getName() {
- return this.name;
- }
private String name;
public void setName(String name) {
this.name = name.trim();
}
public void String getName() {
return this.name;
}
有时候我们很讨厌字符串首尾出现空格, 所以在setter方法中进行了trim处理, 但是这样做的结果带来的副作用会使getter方法的返回值和setter方法不一致, 如果只是将JavaBean当做一个数据容器, 那么最好不要包含任何业务逻辑. 而将业务逻辑放到专门的业务层或者控制层中处理.
正确的做法:
- person.setName(textInput.getText().trim());
person.setName(textInput.getText().trim());
日历对象(Calendar)误用 错误的写法:
- Calendar cal = new GregorianCalender(TimeZone.getTimeZone("Europe/Zurich"));
- cal.setTime(date);
- cal.add(Calendar.HOUR_OF_DAY, 8);
- date = cal.getTime();
Calendar cal = new GregorianCalender(TimeZone.getTimeZone("Europe/Zurich"));
cal.setTime(date);
cal.add(Calendar.HOUR_OF_DAY, 8);
date = cal.getTime();
这里主要是对date, time, calendar和time zone不了解导致. 而在一个时间上增加8小时, 跟time zone没有任何关系, 所以没有必要使用Calendar, 直接用Date对象即可, 而如果是增加天数的话, 则需要使用Calendar, 因为采用不同的时令制可能一天的小时数是不同的(比如有些DST是23或者25个小时) 正确的写法:
- date = new Date(date.getTime() + 8L * 3600L * 1000L); // add 8 hrs
date = new Date(date.getTime() + 8L * 3600L * 1000L); // add 8 hrs
TimeZone的误用 错误的写法:
- Calendar cal = new GregorianCalendar();
- cal.setTime(date);
- cal.set(Calendar.HOUR_OF_DAY, 0);
- cal.set(Calendar.MINUTE, 0);
- cal.set(Calendar.SECOND, 0);
- Date startOfDay = cal.getTime();
Calendar cal = new GregorianCalendar();
cal.setTime(date);
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
Date startOfDay = cal.getTime();
这里有两个错误, 一个是没有没有将毫秒归零, 不过最大的错误是没有指定TimeZone, 不过一般的桌面应用没有问题, 但是如果是服务器端应用则会有一些问题, 比如同一时刻在上海和伦敦就不一样, 因此需要指定的TimeZone. 正确的写法:
- Calendar cal = new GregorianCalendar(user.getTimeZone());
- cal.setTime(date);
- cal.set(Calendar.HOUR_OF_DAY, 0);
- cal.set(Calendar.MINUTE, 0);
- cal.set(Calendar.SECOND, 0);
- cal.set(Calendar.MILLISECOND, 0);
- Date startOfDay = cal.getTime();
Calendar cal = new GregorianCalendar(user.getTimeZone());
cal.setTime(date);
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
Date startOfDay = cal.getTime();
时区(Time Zone)调整的误用 错误的写法:
- public static Date convertTz(Date date, TimeZone tz) {
- Calendar cal = Calendar.getInstance();
- cal.setTimeZone(TimeZone.getTimeZone("UTC"));
- cal.setTime(date);
- cal.setTimeZone(tz);
- return cal.getTime();
- }
public static Date convertTz(Date date, TimeZone tz) {
Calendar cal = Calendar.getInstance();
cal.setTimeZone(TimeZone.getTimeZone("UTC"));
cal.setTime(date);
cal.setTimeZone(tz);
return cal.getTime();
}
这个方法实际上没有改变时间, 输入和输出是一样的. 关于时间的问题可以参考这篇文章: http://www.odi.ch/prog/design/datetime.php 这里主要的问题是Date对象并不包含Time Zone信息. 它总是使用UTC(世界统一时间). 而调用Calendar的getTime/setTime方法会自动在当前时区和UTC之间做转换.
Calendar.getInstance()的误用 错误的写法:
- Calendar c = Calendar.getInstance();
- c.set(2009, Calendar.JANUARY, 15);
Calendar c = Calendar.getInstance();
c.set(2009, Calendar.JANUARY, 15);
Calendar.getInstance()依赖local来选择一个Calendar实现, 不同实现的2009年是不同的, 比如有些Calendar实现就没有January月份. 正确的写法:
- Calendar c = new GregorianCalendar(timeZone);
- c.set(2009, Calendar.JANUARY, 15);
Calendar c = new GregorianCalendar(timeZone);
c.set(2009, Calendar.JANUARY, 15);
Date.setTime()的误用 错误的写法:
- account.changePassword(oldPass, newPass);
- Date lastmod = account.getLastModified();
- lastmod.setTime(System.currentTimeMillis());
account.changePassword(oldPass, newPass);
Date lastmod = account.getLastModified();
lastmod.setTime(System.currentTimeMillis());
在更新密码之后, 修改一下最后更新时间, 这里的用法没有错,但是有更好的做法: 直接传Date对象. 因为Date是Value Object, 不可变的. 如果更新了Date的值, 实际上是生成一个新的Date实例. 这样其他地方用到的实际上不在是原来的对象, 这样可能出现不可预知的异常. 当然这里又涉及到另外一个OO设计的问题, 对外暴露Date实例本身就是不好的做法(一般的做法是在setter方法中设置Date引用参数的clone对象). 另外一种比较好的做法就是直接保存long类型的毫秒数.
正确的做法:
- account.changePassword(oldPass, newPass);
- account.setLastModified(new Date());
account.changePassword(oldPass, newPass);
account.setLastModified(new Date());
SimpleDateFormat非线程安全误用 错误的写法:
- public class Constants {
- public static final SimpleDateFormat date = new SimpleDateFormat("dd.MM.yyyy");
- }
public class Constants {
public static final SimpleDateFormat date = new SimpleDateFormat("dd.MM.yyyy");
}
SimpleDateFormat不是线程安全的. 在多线程并行处理的情况下, 会得到非预期的值. 这个错误非常普遍! 如果真要在多线程环境下公用同一个SimpleDateFormat, 那么做好做好同步(cache flush, lock contention), 但是这样会搞得更复杂, 还不如直接new一个实在.
使用全局参数配置常量类/接口
- public interface Constants {
- String version = "1.0";
- String dateFormat = "dd.MM.yyyy";
- String configFile = ".apprc";
- int maxNameLength = 32;
- String someQuery = "SELECT * FROM ...";
- }
public interface Constants {
String version = "1.0";
String dateFormat = "dd.MM.yyyy";
String configFile = ".apprc";
int maxNameLength = 32;
String someQuery = "SELECT * FROM ...";
}
很多应用都会定义这样一个全局常量类或接口, 但是为什么这种做法不推荐? 因为这些常量之间基本没有任何关联, 只是因为公用才定义在一起. 但是如果其他组件需要使用这些全局变量, 则必须对该常量类产生依赖, 特别是存在server和远程client调用的场景.
比较好的做法是将这些常量定义在组件内部. 或者局限在一个类库内部.
忽略造型溢出(cast overflow) 错误的写法:
- public int getFileSize(File f) {
- long l = f.length();
- return (int) l;
- }
public int getFileSize(File f) {
long l = f.length();
return (int) l;
}
这个方法的本意是不支持传递超过2GB的文件. 最好的做法是对长度进行检查, 溢出时抛出异常. 正确的写法:
- public int getFileSize(File f) {
- long l = f.length();
- if (l > Integer.MAX_VALUE) throw new IllegalStateException("int overflow");
- return (int) l;
- }
public int getFileSize(File f) {
long l = f.length();
if (l > Integer.MAX_VALUE) throw new IllegalStateException("int overflow");
return (int) l;
}
另一个溢出bug是cast的对象不对, 比如下面第一个println. 正确的应该是下面的那个.
- long a = System.currentTimeMillis();
- long b = a + 100;
- System.out.println((int) b-a);
- System.out.println((int) (b-a));
long a = System.currentTimeMillis();
long b = a + 100;
System.out.println((int) b-a);
System.out.println((int) (b-a));
对float和double使用==操作 错误的写法:
- for (float f = 10f; f!=0; f-=0.1) {
- System.out.println(f);
- }
for (float f = 10f; f!=0; f-=0.1) {
System.out.println(f);
}
上面的浮点数递减只会无限接近0而不会等于0, 这样会导致上面的for进入死循环. 通常绝不要对float和double使用==操作. 而采用大于和小于操作. 如果java编译器能针对这种情况给出警告. 或者在java语言规范中不支持浮点数类型的==操作就最好了. 正确的写法:
- for (float f = 10f; f>0; f-=0.1) {
- System.out.println(f);
- }
for (float f = 10f; f>0; f-=0.1) {
System.out.println(f);
}
用浮点数来保存money 错误的写法:
- float total = 0.0f;
- for (OrderLine line : lines) {
- total += line.price * line.count;
- }
- double a = 1.14 * 75; // 85.5 将表示为 85.4999...
- System.out.println(Math.round(a)); // 输出值为85
- BigDecimal d = new BigDecimal(1.14); //造成精度丢失
float total = 0.0f;
for (OrderLine line : lines) {
total += line.price * line.count;
}
double a = 1.14 * 75; // 85.5 将表示为 85.4999...
System.out.println(Math.round(a)); // 输出值为85
BigDecimal d = new BigDecimal(1.14); //造成精度丢失
这个也是一个老生常谈的错误. 比如计算100笔订单, 每笔0.3元, 最终的计算结果是29.9999971. 如果将float类型改为double类型, 得到的结果将是30.000001192092896. 出现这种情况的原因是, 人类和计算的计数方式不同. 人类采用的是十进制, 而计算机是二进制.二进制对于计算机来说非常好使, 但是对于涉及到精确计算的场景就会带来误差. 比如银行金融中的应用.
因此绝不要用浮点类型来保存money数据. 采用浮点数得到的计算结果是不精确的. 即使与int类型做乘法运算也会产生一个不精确的结果.那是因为在用二进制存储一个浮点数时已经出现了精度丢失. 最好的做法就是用一个string或者固定点数来表示. 为了精确, 这种表示方式需要指定相应的精度值.
BigDecimal就满足了上面所说的需求. 如果在计算的过程中精度的丢失超出了给定的范围, 将抛出runtime exception. 正确的写法:
- BigDecimal total = BigDecimal.ZERO;
- for (OrderLine line : lines) {
- BigDecimal price = new BigDecimal(line.price);
- BigDecimal count = new BigDecimal(line.count);
- total = total.add(price.multiply(count)); // BigDecimal is immutable!
- }
- total = total.setScale(2, RoundingMode.HALF_UP);
- BigDecimal a = (new BigDecimal("1.14")).multiply(new BigDecimal(75)); // 85.5 exact
- a = a.setScale(0, RoundingMode.HALF_UP); // 86
- System.out.println(a); // correct output: 86
- BigDecimal a = new BigDecimal("1.14");
BigDecimal total = BigDecimal.ZERO;
for (OrderLine line : lines) {
BigDecimal price = new BigDecimal(line.price);
BigDecimal count = new BigDecimal(line.count);
total = total.add(price.multiply(count)); // BigDecimal is immutable!
}
total = total.setScale(2, RoundingMode.HALF_UP);
BigDecimal a = (new BigDecimal("1.14")).multiply(new BigDecimal(75)); // 85.5 exact
a = a.setScale(0, RoundingMode.HALF_UP); // 86
System.out.println(a); // correct output: 86
BigDecimal a = new BigDecimal("1.14");
不使用finally块释放资源 错误的写法:
- public void save(File f) throws IOException {
- OutputStream out = new BufferedOutputStream(new FileOutputStream(f));
- out.write(...);
- out.close();
- }
- public void load(File f) throws IOException {
- InputStream in = new BufferedInputStream(new FileInputStream(f));
- in.read(...);
- in.close();
- }
public void save(File f) throws IOException {
OutputStream out = new BufferedOutputStream(new FileOutputStream(f));
out.write(...);
out.close();
}
public void load(File f) throws IOException {
InputStream in = new BufferedInputStream(new FileInputStream(f));
in.read(...);
in.close();
}
上面的代码打开一个文件输出流, 操作系统为其分配一个文件句柄, 但是文件句柄是一种非常稀缺的资源, 必须通过调用相应的close方法来被正确的释放回收. 而为了保证在异常情况下资源依然能被正确回收, 必须将其放在finally block中. 上面的代码中使用了BufferedInputStream将file stream包装成了一个buffer stream, 这样将导致在调用close方法时才会将buffer stream写入磁盘. 如果在close的时候失败, 将导致写入数据不完全. 而对于FileInputStream在finally block的close操作这里将直接忽略.
如果BufferedOutputStream.close()方法执行顺利则万事大吉, 如果失败这里有一个潜在的bug(http://bugs.sun.com/view_bug.do?bug_id=6335274): 在close方法内部调用flush操作的时候, 如果出现异常, 将直接忽略. 因此为了尽量减少数据丢失, 在执行close之前显式的调用flush操作.
下面的代码有一个小小的瑕疵: 如果分配file stream成功, 但是分配buffer stream失败(OOM这种场景), 将导致文件句柄未被正确释放. 不过这种情况一般不用担心, 因为JVM的gc将帮助我们做清理.
- // code for your cookbook
- public void save() throws IOException {
- File f = ...
- OutputStream out = new BufferedOutputStream(new FileOutputStream(f));
- try {
- out.write(...);
- out.flush(); // don't lose exception by implicit flush on close
- } finally {
- out.close();
- }
- }
- public void load(File f) throws IOException {
- InputStream in = new BufferedInputStream(new FileInputStream(f));
- try {
- in.read(...);
- } finally {
- try { in.close(); } catch (IOException e) { }
- }
- }
// code for your cookbook
public void save() throws IOException {
File f = ...
OutputStream out = new BufferedOutputStream(new FileOutputStream(f));
try {
out.write(...);
out.flush(); // don't lose exception by implicit flush on close
} finally {
out.close();
}
}
public void load(File f) throws IOException {
InputStream in = new BufferedInputStream(new FileInputStream(f));
try {
in.read(...);
} finally {
try { in.close(); } catch (IOException e) { }
}
}
数据库访问也涉及到类似的情况:
- Car getCar(DataSource ds, String plate) throws SQLException {
- Car car = null;
- Connection c = null;
- PreparedStatement s = null;
- ResultSet rs = null;
- try {
- c = ds.getConnection();
- s = c.prepareStatement("select make, color from cars where plate=?");
- s.setString(1, plate);
- rs = s.executeQuery();
- if (rs.next()) {
- car = new Car();
- car.make = rs.getString(1);
- car.color = rs.getString(2);
- }
- } finally {
- if (rs != null) try { rs.close(); } catch (SQLException e) { }
- if (s != null) try { s.close(); } catch (SQLException e) { }
- if (c != null) try { c.close(); } catch (SQLException e) { }
- }
- return car;
- }
Car getCar(DataSource ds, String plate) throws SQLException {
Car car = null;
Connection c = null;
PreparedStatement s = null;
ResultSet rs = null;
try {
c = ds.getConnection();
s = c.prepareStatement("select make, color from cars where plate=?");
s.setString(1, plate);
rs = s.executeQuery();
if (rs.next()) {
car = new Car();
car.make = rs.getString(1);
car.color = rs.getString(2);
}
} finally {
if (rs != null) try { rs.close(); } catch (SQLException e) { }
if (s != null) try { s.close(); } catch (SQLException e) { }
if (c != null) try { c.close(); } catch (SQLException e) { }
}
return car;
}
finalize方法误用 错误的写法:
- public class FileBackedCache {
- private File backingStore;
- ...
- protected void finalize() throws IOException {
- if (backingStore != null) {
- backingStore.close();
- backingStore = null;
- }
- }
- }
public class FileBackedCache {
private File backingStore;
...
protected void finalize() throws IOException {
if (backingStore != null) {
backingStore.close();
backingStore = null;
}
}
}
这个问题Effective Java这本书有详细的说明. 主要是finalize方法依赖于GC的调用, 其调用时机可能是立马也可能是几天以后, 所以是不可预知的. 而JDK的API文档中对这一点有误导: 建议在该方法中来释放I/O资源.
正确的做法是定义一个close方法, 然后由外部的容器来负责调用释放资源.
- public class FileBackedCache {
- private File backingStore;
- ...
- public void close() throws IOException {
- if (backingStore != null) {
- backingStore.close();
- backingStore = null;
- }
- }
- }
public class FileBackedCache {
private File backingStore;
...
public void close() throws IOException {
if (backingStore != null) {
backingStore.close();
backingStore = null;
}
}
}
在JDK 1.7 (Java 7)中已经引入了一个AutoClosable接口. 当变量(不是对象)超出了try-catch的资源使用范围, 将自动调用close方法.
- try (Writer w = new FileWriter(f)) { // implements Closable
- w.write("abc");
- // w goes out of scope here: w.close() is called automatically in ANY case
- } catch (IOException e) {
- throw new RuntimeException(e.getMessage(), e);
- }
try (Writer w = new FileWriter(f)) { // implements Closable
w.write("abc");
// w goes out of scope here: w.close() is called automatically in ANY case
} catch (IOException e) {
throw new RuntimeException(e.getMessage(), e);
}
Thread.interrupted方法误用 错误的写法:
- try {
- Thread.sleep(1000);
- } catch (InterruptedException e) {
- // ok
- }
- or
- while (true) {
- if (Thread.interrupted()) break;
- }
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// ok
}
or
while (true) {
if (Thread.interrupted()) break;
}
这里主要是interrupted静态方法除了返回当前线程的中断状态, 还会将当前线程状态复位. 正确的写法:
- try {
- Thread.sleep(1000);
- } catch (InterruptedException e) {
- Thread.currentThread().interrupt();
- }
- or
- while (true) {
- if (Thread.currentThread().isInterrupted()) break;
- }
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
or
while (true) {
if (Thread.currentThread().isInterrupted()) break;
}
在静态变量初始化时创建线程 错误的写法:
- class Cache {
- private static final Timer evictor = new Timer();
- }
class Cache {
private static final Timer evictor = new Timer();
}
Timer构造器内部会new一个thread, 而该thread会从它的父线程(即当前线程)中继承各种属性. 比如context classloader, threadlocal以及其他的安全属性(访问权限). 而加载当前类的线程可能是不确定的, 比如一个线程池中随机的一个线程. 如果你需要控制线程的属性, 最好的做法就是将其初始化操作放在一个静态方法中, 这样初始化将由它的调用者来决定.
正确的做法:
- class Cache {
- private static Timer evictor;
- public static setupEvictor() {
- evictor = new Timer();
- }
- }
class Cache {
private static Timer evictor;
public static setupEvictor() {
evictor = new Timer();
}
}
已取消的定时器任务依然持有状态 错误的写法:
- final MyClass callback = this;
- TimerTask task = new TimerTask() {
- public void run() {
- callback.timeout();
- }
- };
- timer.schedule(task, 300000L);
- try {
- doSomething();
- } finally {
- task.cancel();
- }
final MyClass callback = this;
TimerTask task = new TimerTask() {
public void run() {
callback.timeout();
}
};
timer.schedule(task, 300000L);
try {
doSomething();
} finally {
task.cancel();
}
上面的task内部包含一个对外部类实例的应用, 这将导致该引用可能不会被GC立即回收. 因为Timer将保留TimerTask在指定的时间之后才被释放. 因此task对应的外部类实例将在5分钟后被回收. 正确的写法:
- TimerTask task = new Job(this);
- timer.schedule(task, 300000L);
- try {
- doSomething();
- } finally {
- task.cancel();
- }
- static class Job extends TimerTask {
- private MyClass callback;
- public Job(MyClass callback) {
- this.callback = callback;
- }
- public boolean cancel() {
- callback = null;
- return super.cancel();
- }
- public void run() {
- if (callback == null) return;
- callback.timeout();
- }
- }