配置FindBugs和常见FindBugs错误

配置FindBugs:

在这里可以对FindBugs规则等进行详细设置。

　　选择你的项目，右键 => Properties => FindBugs =>

1 Run Automatically开关

当此项选中后，FindBugs将会在你修改Java类时自动运行，如你设置了Eclipse自动编译开关后，当你修改完Java文件保存，FindBugs就会运行，并将相应的信息显示出来。

当此项没有选中，你只能每次在需要的时候自己去运行FindBugs来检查你的代码。

2 Detector Configuration选择项

在这里你可以选择所要进行检查的相关的Bug Pattern条目,你可以根据需要选择或去掉相应的 检查条件。

3 Minimum priority to report选择项

这个选择项是让你选择哪个级别的信息进行显示，有Low、Medium、High三个选择项可以选择，很类似于Log4J的级别设置啦。 比如：

你选择了High选择项，那么只有是High级别的提示信息才会被显示。

你选择了Medium选择项，那么只有是Medium和High级别的提示信息才会被显示。

你选择了Low选择项，那么所有级别的提示信息都会被显示。

4 Report bug categories选择项

在这里是一些显示Bug分类的选择：

Malicious code vulnerability关于恶意破坏代码相关方面的

Correctness关于代码正确性相关方面的

Internationalization关于代码国际化相关方面的

Performance关于代码性能相关方面的

Multithreaded correctness关于代码多线程正确性相关方面的

另外FindBugs有UI页面，可以单独运行。也可以通过Ant以及命令行方式运行。

其它分析工具

除FingBugs静态分析工具外，还有PMD和Checkstyle，FingBugs、PMD和Checkstyle三个工具各有不同的特点，联合使用有助于减少误报错误，提高报告的准确率。

这三个工具检查的侧重点各有不同：

工具

目的

主要检查内容

FindBugs

基于Bug Patterns概念，查找java bytecode中的潜在bug。在目前版本中，它不检查java源文件。 主要检查bytecode中的bug patterns，也允许用户自定义特定的bug patterns。

 PMD

检查java源文件中的潜在问题。

主要包括：

-  空try/catch/finally/switch语句块

-       未使用的局部变量、参数和private方法

-       空if/while语句

-       过于复杂的表达式，如不必要的if语句等

-       复杂类

CheckStyle

检查java源文件是否与代码规范相符

主要包括

-       Javadoc注释

-       命名规范

-       Headers

-       Imports

-       Size冲突和度量，如过长的方法

-       Whitespace

-       Modifiers

-       Blocks

-       Coding Problems

-       Class Design

-       重复代码

-       Miscellaneous Checks

-       Optional Checks

配套的Bug解释模式

为了有针对性的使用这个工具，减少bug的误报，提高使用效率，我们选择了10个左右的bug模式，下面就是对这10个模式的解释。

这些bug可能会引起程序的性能或逻辑问题.

需要说明的是,findbugs能检测的bug pattern远不仅于此,甚至可以定制自己的探测器,因此,这个文档会不断扩充,同时,也欢迎大家不断探索和分享使用实践.

大的分类主要包括以下几种：

Bad practice	不好的习惯
Correctness	代码的正确性
Dodgy	小问题
Malicious code vulnerability	恶意代码
Internationalization	国际化问题
Performance	性能问题
Security	安全性问题
Multithreaded currectness	线程问题
Experrimental	实验性问题

API：http://findbugs.sourceforge.net/api/index.html

技术手册：http://findbugs.sourceforge.net/manual/index.html

更多请参见官网：http://findbugs.sourceforge.net/bugDescriptions.html

6.1、       ES_COMPARING_PARAMETER_STRING_WITH_EQ

ES: Comparison of String parameter using == or != (ES_COMPARING_PARAMETER_STRING_WITH_EQ)

This code compares a java.lang.String parameter for reference equality using the == or != operators. Requiring callers to pass only String constants or interned strings to a method is unnecessarily fragile, and rarely leads to measurable performance gains. Consider using the equals(Object) method instead.

使用 == 或者 != 来比较字符串或interned字符串，不会获得显著的性能提升，同时并不可靠，请考虑使用equals()方法。

6.2、       HE_EQUALS_NO_HASHCODE

HE: Class defines equals() but not hashCode() (HE_EQUALS_NO_HASHCODE)

This class overrides equals(Object), but does not override hashCode().  Therefore, the class may violate the invariant that equal objects must have equal hashcodes.

类定义了equals()方法但没有重写hashCode()方法，这样违背了相同对象必须具有相同的hashcodes的原则

6.3、       IT_NO_SUCH_ELEMENT

It: Iterator next() method can't throw NoSuchElement exception (IT_NO_SUCH_ELEMENT)

This class implements the java.util.Iterator interface.  However, its next() method is not capable of throwing java.util.NoSuchElementException.  The next() method should be changed so it throws NoSuchElementException if is called when there are no more elements to return.

迭代器Iterator无法抛出NoSuchElement异常，类实现了java.util.Iterator接口，但是next()方法无法抛出java.util.NoSuchElementException异常，因此，next()方法应该做如此修改，当被调用时，如果没有element返回，则抛出NoSuchElementException异常

6.4、       J2EE_STORE_OF_NON_SERIALIZABLE_OBJECT_INTO_SESSION

J2EE: Store of non serializable object into HttpSession (J2EE_STORE_OF_NON_SERIALIZABLE_OBJECT_INTO_SESSION)

This code seems to be storing a non-serializable object into an HttpSession. If this session is passivated or migrated, an error will result.

将没有实现serializable的对象放到HttpSession中，当这个session被钝化和迁移时，将会产生错误，建议放到HttpSession中的对象都实现serializable接口。

6.5、       ODR_OPEN_DATABASE_RESOURCE

ODR: Method may fail to close database resource (ODR_OPEN_DATABASE_RESOURCE)

The method creates a database resource (such as a database connection or row set), does not assign it to any fields, pass it to other methods, or return it, and does not appear to close the object on all paths out of the method.  Failure to close database resources on all paths out of a method may result in poor performance, and could cause the application to have problems communicating with the database.

方法可能未关闭数据库资源，未关闭数据库资源将会导致性能变差，还可能引起应用与服务器间的通讯问题。

6.6、       OS_OPEN_STREAM

OS: Method may fail to close stream (OS_OPEN_STREAM)

The method creates an IO stream object, does not assign it to any fields, pass it to other methods that might close it, or return it, and does not appear to close the stream on all paths out of the method.  This may result in a file descriptor leak.  It is generally a good idea to use a finally block to ensure that streams are closed.

方法可能未关闭stream，方法产生了一个IO流，却未关闭，将会导致文件描绘符的泄漏，建议使用finally block来确保io stream被关闭。

6.7、       DMI_CALLING_NEXT_FROM_HASNEXT

DMI: hasNext method invokes next (DMI_CALLING_NEXT_FROM_HASNEXT)

The hasNext() method invokes the next() method. This is almost certainly wrong, since the hasNext() method is not supposed to change the state of the iterator, and the next method is supposed to change the state of the iterator.

6.8、       IL_INFINITE_LOOP

IL: An apparent infinite loop (IL_INFINITE_LOOP)

This loop doesn't seem to have a way to terminate (other than by perhaps throwing an exception).

明显的无限循环.

6.9、       IL_INFINITE_RECURSIVE_LOOP

IL: An apparent infinite recursive loop (IL_INFINITE_RECURSIVE_LOOP)

This method unconditionally invokes itself. This would seem to indicate an infinite recursive loop that will result in a stack overflow.

明显的无限迭代循环,将导致堆栈溢出.

6.10、   WMI_WRONG_MAP_ITERATOR

WMI: Inefficient use of keySet iterator instead of entrySet iterator (WMI_WRONG_MAP_ITERATOR)

This method accesses the value of a Map entry, using a key that was retrieved from a keySet iterator. It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.

使用了keySet iterator和Map.get(key)来获取Map值,这种方式效率低,建议使用entrySet的iterator效率更高.

6.11、   IM_BAD_CHECK_FOR_ODD

IM: Check for oddness that won't work for negative numbers (IM_BAD_CHECK_FOR_ODD)

The code uses x % 2 == 1 to check to see if a value is odd, but this won't work for negative numbers (e.g., (-5) % 2 == -1). If this code is intending to check for oddness, consider using x & 1 == 1, or x % 2 != 0.

奇偶检测逻辑,未考虑负数情况.

7.实际项目中Bug类型统计

7.1、       Call to equals() comparing different types

id: EC_UNRELATED_TYPES, type: EC, category: CORRECTNESS
This method calls equals(Object) on two references
of different class types with no common subclasses. Therefore, the objects
being compared are unlikely to be members of the same class at runtime (unless
some application classes were not analyzed, or dynamic class loading can occur
at runtime). According to the contract of equals(), objects of different
classes should always compare as unequal; therefore, according to the contract
defined by java.lang.Object.equals(Object), the result of this comparison will
always be false at runtime.

原因分析：

这缺陷的意思是，大部分都是类型永远不会有这种情况， 比如a为DOUBLE类型所以EQUALS只匹配字符串
if(a.equals())或if(a.quals())这类判断是根本不会有用的；

示例：if("1".equals(DAOValue.valueofSuccess()))

7.2、       Dead store to local
variable

id: DLS_DEAD_LOCAL_STORE, type: DLS, category: STYLE

This
instruction assigns a value to a local variable, but the value is not read or
used in any subsequent instruction. Often, this indicates an error, because the
value computed is never used.

Note
that Sun's javac compiler often generates dead stores for final local
variables. Because FindBugs is a bytecode-based tool, there is no easy way to
eliminate these false positives.

原因分析：

DLS问题指的是给本地变量赋了一个值，但随后的代码并没有用到这个值。

7.3、       Method call passes null
for nonnull parameter

id: NP_NULL_PARAM_DEREF, type: NP, category: CORRECTNESS

This
method call passes a null value for a nonnull method parameter. Either the
parameter is annotated as a parameter that should always be nonnull, or
analysis has shown that it will always be dereferenced.

原因分析：对参数为null的情况未作处理。

例如：

7.4、       Method with Boolean return
type returns explicit null

id:
NP_BOOLEAN_RETURN_NULL, type: NP, category: BAD_PRACTICE

A
method that returns either Boolean.TRUE, Boolean.FALSE or null is an accident
waiting to happen. This method can be invoked as though it returned a value of
type boolean, and the compiler will insert automatic unboxing of the Boolean
value. If a null value is returned, this will result in a NullPointerException.

原因分析：

方法如果定义为返回类型Boolean，则可以返回Boolean.TRUE,
Boolean.FALSE or null （如果 return 的是 true or  false， 则AutoBoxing 成 Boolean.TRUE,
Boolean.FALSE）。因为JDK 支持
基本类型和装箱类型的自动转化， 所以下面的代码中：

boolean
result = test_NP_BOOLEAN_RETURN_NULL();

因为此时test_NP_BOOLEAN_RETURN_NULL() 返回的是NULL， 所以 JDK 做 automatic unboxing 的操作时， 即调用了 object. booleanValue() 方法时，抛出了空指针。

改成：boolean result = test_NP_BOOLEAN_RETURN_NULL()==null?false:true;

7.5、       No relationship between
generic parameter and method argument

id: GC_UNRELATED_TYPES, type: GC, category: CORRECTNESS

This
call to a generic collection method contains an argument with an incompatible
class from that of the collection's parameter (i.e., the type of the argument
is neither a supertype nor a subtype of the corresponding generic type
argument). Therefore, it is unlikely that the collection contains any objects
that are equal to the method argument used here. Most likely, the wrong value
is being passed to the method.

In
general, instances of two unrelated classes are not equal. For example, if the
Foo and Bar classes are not related by subtyping, then an instance of Foo
should not be equal to an instance of Bar. Among other issues, doing so will
likely result in an equals method that is not symmetrical. For example, if you
define the Foo class so that a Foo can be equal to a String, your equals method
isn't symmetrical since a String can only be equal to a String.

In
rare cases, people do define nonsymmetrical equals methods and still manage to
make their code work. Although none of the APIs document or guarantee it, it is
typically the case that if you check if a Collection<String> contains a
Foo, the equals method of argument (e.g., the equals method of the Foo class)
used to perform the equality checks.

原因分析：调用Collection类中的contains方法比较时，所比较的两个参数类型不致；

例如：

修改后：

7.6、       Null pointer dereference
in method on exception path

id:
NP_ALWAYS_NULL_EXCEPTION, type: NP, category: CORRECTNESS

A
pointer which is null on an exception path is dereferenced here.  This will lead to a NullPointerException when
the code is executed.  Note that because
FindBugs currently does not prune infeasible exception paths, this may be a
false warning.

Also
note that FindBugs considers the default case of a switch statement to be an
exception path, since the default case is often infeasible.

原因分析：在异常处理时，调用一个空对象的方法时可能引起空指针异常。

例如：

7.7、       Nullcheck of value
previously dereferenced

id:
RCN_REDUNDANT_NULLCHECK_WOULD_HAVE_BEEN_A_NPE, type: RCN, category:
CORRECTNESS

A
value is checked here to see whether it is null, but this value can't be null
because it was previously dereferenced and if it were null a null pointer
exception would have occurred at the earlier dereference. Essentially, this
code and the previous dereference disagree as to whether this value is allowed
to be null. Either the check is redundant or the previous dereference is
erroneous.

原因分析：前面获取的对象，现在引用的时候没有交验是否为null。

例如：

7.8、       Possible null pointer
dereference

id: NP_NULL_ON_SOME_PATH, type: NP, category: CORRECTNESS

There
is a branch of statement that, if
executed, guarantees that a null value will be dereferenced, which would
generate a NullPointerException when the code is executed. Of course, the
problem might be that the branch or statement is infeasible and that the null
pointer exception can't ever be executed; deciding that is beyond the ability
of FindBugs.

原因分析：可能存在空引用。

例如：

7.9、       Possible null pointer
dereference in method on exception path

id:
NP_NULL_ON_SOME_PATH_EXCEPTION, type:
NP, category: CORRECTNESS

A
reference value which is null on some exception control path is dereferenced
here.  This may lead to a
NullPointerException when the code is executed. 
Note that because FindBugs currently does not prune infeasible exception
paths, this may be a false warning.

Also
note that FindBugs considers the default case of a switch statement to be an
exception path, since the default case is often infeasible.

原因分析：

代码调用时，
遇到异常分支， 可能造成一个对象没有获得赋值依旧保持NULL空指针。
接下来如果对这个对象有引用， 可能造成NullPointerException
空指针异常。

例如：

7.10、   Test for floating point
equality

id:
FE_FLOATING_POINT_EQUALITY, type:
FE, category: STYLE

This
operation compares two floating point values for equality. Because floating
point calculations may involve rounding, calculated float and double values may
not be accurate. For values that must be precise, such as monetary values,
consider using a fixed-precision type such as BigDecimal. For values that need
not be precise, consider comparing for equality within some range, for example:
if ( Math.abs(x - y) < .0000001 ). See the Java Language Specification,
section 4.2.4.

原因分析：

Float类型的数据比较时，会存在的定的误差值，用!=来比较不是很准确，建议比较两个数的绝对值是否在一定的范围内来进行比较。如，if ( Math.abs(x - y) < .0000001 )

例如：

7.11、   Useless assignment in
return statement

id:
DLS_DEAD_LOCAL_STORE_IN_RETURN, type:
DLS, category: STYLE

This
statement assigns to a local variable in a return statement. This assignment
has effect. Please verify that this statement does the right thing.

原因分析：

在return的对象中，没有必要通过对象赋值再进行返回。

例如：

7.12、   Write to static field from
instance method

id:
ST_WRITE_TO_STATIC_FROM_INSTANCE_METHOD, type:
ST, category: STYLE

This
instance method writes to a static field. This is tricky to get correct if
multiple instances are being manipulated, and generally bad practice.

原因分析：向static字段中写入值。

例如：
 private static DBRBO dbrBO;
 public final void refresh() {
        danskeBankBO = null;
        dbrBO = null;
        fileAndPathBO = null;
    }
建议改为：去掉static。

7.13、   Incorrect lazy
initialization and update of static field

id:
LI_LAZY_INIT_UPDATE_STATIC, type:
LI, category: MT_CORRECTNESS

This
method contains an unsynchronized lazy initialization of a static field. After
the field is set, the object stored into that location is further updated or
accessed. The setting of the field is visible to other threads as soon as it is
set. If the futher accesses in the method that set the field serve to
initialize the object, then you have a very
serious multithreading bug, unless something else prevents any other thread
from accessing the stored object until it is fully initialized.

Even
if you feel confident that the method is never called by multiple threads, it
might be better to not set the static field until the value you are setting it
to is fully populated/initialized.

原因分析：

该方法的初始化中包含了一个迟缓初始化的静态变量。你的方法引用了一个静态变量，估计是类静态变量，那么多线程调用这个方法时，你的变量就会面临线程安全的问题了，除非别的东西阻止任何其他线程访问存储对象从直到它完全被初始化。

7.14、   Method ignores return
value

id:
RV_RETURN_VALUE_IGNORED, type: RV, category: CORRECTNESS

The
return value of this method should be checked. One common cause of this warning
is to invoke a method on an immutable object, thinking that it updates the
object. For example, in the following code fragment,

String
dateString = getHeaderField(name);

dateString.trim();

the
programmer seems to be thinking that the trim() method will update the String
referenced by dateString. But since Strings are immutable, the trim() function
returns a new String value, which is being ignored here. The code should be
corrected to:

String
dateString = getHeaderField(name);

dateString
= dateString.trim();

原因分析：方法忽略了设置返回值。

例如：

7.15、   Method might ignore
exception

id: DE_MIGHT_IGNORE, type: DE, category: BAD_PRACTICE

This
method might ignore an exception.  In
general, exceptions should be handled or reported in some way, or they should
be thrown out of the method.

原因分析：应该将异常
处理、打印或者抛出

例如：

7.16、   Unwritten field

id: UWF_UNWRITTEN_FIELD, type: UwF, category: CORRECTNESS

This
field is never written.  All reads of it
will return the default value. Check for errors (should it have been
initialized?), or remove it if it is useless.

原因分析：从未被初始化的变量，调用它时，将返回默认值，要么初始化，要么删掉它。

例如：

7.17、   Value is null and
guaranteed to be dereferenced on exception path

id:
NP_GUARANTEED_DEREF_ON_EXCEPTION_PATH, type:
NP, category: CORRECTNESS

There
is a statement or branch on an exception path that if executed guarantees that
a value is null at this point, and that value that is guaranteed to be
dereferenced (except on forward paths involving runtime exceptions).

原因分析：exception分支上，存在引用一个null对象的方法，引发空指针异常。

例如：

7.18、  
Very
confusing method names

id: NM_VERY_CONFUSING, type: Nm, category: CORRECTNESS

The
referenced methods have names that differ only by capitalization. This is very
confusing because if the capitalization were identical then one of the methods
would override the other.

原因分析：被引用的方法中存在容易混淆的变量。

例如：fzgsdm改成 fzgsDm 即可。

7.19、   Method invokes inefficient
new String() constructor

id: DM_STRING_VOID_CTOR, type: Dm, category: Performance
Creating a new java.lang.String object
using the no-argument constructor wastes memory because the object so created
will be functionally indistinguishable from the empty string constant
"".  Java guarantees that identical string constants will be
represented by the same String object.  Therefore, you should just use the
empty string constant directly.

原因分析：不使用new String()定义空的字符串

例如：

7.20、  
Load
of known null value

id: NP_LOAD_OF_KNOWN_NULL_VALUE, type: Np, category: Dodgy

The variable referenced at this point is
known to be null due to an earlier check against null. Although this is valid,
it might be a mistake (perhaps you intended to refer to a different variable,
or perhaps the earlier check to see if the variable is null should have been a
check to see if it was nonnull).

原因分析：null值的不当使用。

例如：

7.21、  
Method
concatenates strings using + in a loop  

id: SBSC_USE_STRINGBUFFER_CONCATENATION, type: SBSC, category: Performance

The method seems
to be building a String using concatenation in a loop. In each iteration, the
String is converted to a StringBuffer/StringBuilder, appended to, and converted
back to a String. This can lead to a cost quadratic in the number of
iterations, as the growing string is recopied in each iteration. Better
performance can be obtained by using a StringBuffer (or StringBuilder in Java
1.5) explicitly.

For example:

// This is bad

String s = "";

for (int i = 0; i <
field.length; ++i) {

s = s + field[i];

}

// This is better

StringBuffer buf = new
StringBuffer();

for (int i = 0; i <
field.length; ++i) {

buf.append(field[i]);

}

String s = buf.toString();

原因分析：在循环里使用字符串连接，效率低，应该使用StringBuilder/StringBuffer