最近在项目里面遇到了很多比较奇葩的问题,一段时间的挣扎之后发现都是基础不牢,地动山摇的结果.Java和Android的源码看的还是比较少的原因.今天是将最近遇到的几个比较困扰我时间长的问题列出来.聊以慰藉.至少这几天的付出是有回报的.
- java.lang.UnsupportedOperationException
org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: java.lang.UnsupportedOperationException
### The error may exist in kr/co/goodwilldd/board/mapper/CommonBoardMapper.java (best guess)
### The error may involve defaultParameterMap
### The error occurred while setting parameters
### SQL: SELECT * FROM TB_BOARD_DATA
### Cause: java.lang.UnsupportedOperationException
at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:73)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:364)
at com.sun.proxy.$Proxy13.selectList(Unknown Source)
at org.mybatis.spring.SqlSessionTemplate.selectList(SqlSessionTemplate.java:194)
at org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:114)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:58)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:43)
at com.sun.proxy.$Proxy25.getList(Unknown Source)
at kr.co.goodwilldd.board.CommonBoardTest.getListByMapper(CommonBoardTest.java:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:88)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: java.lang.UnsupportedOperationException
### The error may exist in kr/co/goodwilldd/board/mapper/CommonBoardMapper.java (best guess)
### The error may involve defaultParameterMap
### The error occurred while setting parameters
### SQL: SELECT * FROM TB_BOARD_DATA
### Cause: java.lang.UnsupportedOperationException
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:23)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:107)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:98)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:354)
... 36 more
Caused by: java.lang.UnsupportedOperationException
at org.apache.ibatis.reflection.wrapper.CollectionWrapper.findProperty(CollectionWrapper.java:42)
at org.apache.ibatis.reflection.MetaObject.findProperty(MetaObject.java:76)
at org.apache.ibatis.executor.resultset.FastResultSetHandler.applyAutomaticMappings(FastResultSetHandler.java:342)
at org.apache.ibatis.executor.resultset.FastResultSetHandler.getRowValue(FastResultSetHandler.java:267)
at org.apache.ibatis.executor.resultset.FastResultSetHandler.handleRowValues(FastResultSetHandler.java:216)
at org.apache.ibatis.executor.resultset.FastResultSetHandler.handleResultSet(FastResultSetHandler.java:188)
at org.apache.ibatis.executor.resultset.FastResultSetHandler.handleResultSets(FastResultSetHandler.java:154)
at org.apache.ibatis.executor.statement.PreparedStatementHandler.query(PreparedStatementHandler.java:57)
at org.apache.ibatis.executor.statement.RoutingStatementHandler.query(RoutingStatementHandler.java:70)
at org.apache.ibatis.executor.ReuseExecutor.doQuery(ReuseExecutor.java:54)
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:259)
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:132)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:105)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:81)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:104)
... 42 more遇到这个问题,一开始我也是懵了,但是我发现我的代码的逻辑写的没有问题的时候我就开始出现了疑惑.根据报错的地方我仔细的看了又看发现真的没有问题,于是我看了一下源码
以下是官方源码的内容:
/**
* Returns a fixed-size list backed by the specified array. (Changes to
* the returned list "write through" to the array.) This method acts
* as bridge between array-based and collection-based APIs, in
* combination with {@link Collection#toArray}. The returned list is
* serializable and implements {@link RandomAccess}.
*
* <p>This method also provides a convenient way to create a fixed-size
* list initialized to contain several elements:
* <pre>
* List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");
* </pre>
*
* @param a the array by which the list will be backed
* @return a list view of the specified array
*/
@SafeVarargs
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);
}这段代码看完之后就会发现逻辑很简单就是将一个array转化成一个有固定大小(fixed-size)的List集合,但是为什么我的代码会出现错误呢但是为什么我的代码会出现错误呢,接下来我们就来看一下我们的返回的数据—return new ArrayList<>(a);创建的新的对象到底是什么鬼.
以下是官方在这个Arrays里面的关于ArrayList的代码块
/**
* @serial include
*/
private static class ArrayList<E> extends AbstractList<E>
implements RandomAccess, java.io.Serializable
{
private static final long serialVersionUID = -2764017481108945198L;
private final E[] a;
ArrayList(E[] array) {
if (array==null)
throw new NullPointerException();
a = array;
}
public int size() {
return a.length;
}
public Object[] toArray() {
return a.clone();
}
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
return Arrays.copyOf(this.a, size,
(Class<? extends T[]>) a.getClass());
System.arraycopy(this.a, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
public E get(int index) {
return a[index];
}
public E set(int index, E element) {
E oldValue = a[index];
a[index] = element;
return oldValue;
}
public int indexOf(Object o) {
if (o==null) {
for (int i=0; i<a.length; i++)
if (a[i]==null)
return i;
} else {
for (int i=0; i<a.length; i++)
if (o.equals(a[i]))
return i;
}
return -1;
}
public boolean contains(Object o) {
return indexOf(o) != -1;
}
}看完之后有什么感想呢????就是Arrays$ArraysList它是继承了AbstractList的,那么我们再到源码里面看一下AbstractList的实现
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {......}我们发现AbstractList是一个抽象类,里面有几个方法,例如:
/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* {@code UnsupportedOperationException}.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
throw new UnsupportedOperationException();
}
/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* {@code UnsupportedOperationException}.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* {@code UnsupportedOperationException}.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
throw new UnsupportedOperationException();
}嗯,看到了什么,就是在这三个方法里面,具体的实现都是抛出一个共同的异常new UnsupportedOperationException()..因为我们的Arrays$ArrayList只是继承了AbstractList但是没有对这三个方法进行重写,所以当我们在调用这三个方法的时候就会直接抛出这个异常了.
解决方法
在java.util.ArrayList的构造方法中有一个构造提供了解决的方法,
具体官方代码如下:
/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}这个就是将一个Collection类型的集合添加到当前的ArrayList集合里面
2.在Android界面上点击事件冲突的一种情况
在listview的显示上面,如果是item本身要相应用户的点击事件,此外在item上面的button按钮也要相应一个点击事件,这个时候我们通常需要用的就是一个属性android:descendantFocusability
官方api是这么表示的:
该属性是当一个为view获取焦点时，定义viewGroup和其子控件两者之间的关系。
属性的值有三种：
beforeDescendants：viewgroup会优先其子类控件而获取到焦点
afterDescendants：viewgroup只有当其子类控件不需要获取焦点时才获取焦点
blocksDescendants：viewgroup会覆盖子类控件而直接获得焦点
通常我们用到的是第三种，即在Item布局的根布局加上android:descendantFocusability=”blocksDescendants”的属性就好了。
3.关于前端JS的几个小坑
由于对方法的执行顺序没有搞懂,导致了一开始写的代码里面,在调用方法的时候出现了一些不一样的情况.我将复杂的代码简单化,来展示这个bug
第一种情况
eat();
function eat(){
alert(1);
}
var eat = function(){
alert(2);
}第二种情况
function eat(){
alert(1);
}
eat();
var eat = function(){
alert(2);
}
第三种情况
function eat(){
alert(1);
}
var eat = function(){
alert(2);
}
eat();
在这里,我可以说,前面两种情况,弹出的结果都是 1 , 但是最后一种情况结果就是 2 .
这个是为什么呢??????????
答案就在两点之间:①JS代码的执行顺序 ②我称之为内存中选择的优先级
首先来说第一种:JS的代码的执行顺序,我们只需要记得定义优先于执行(即 var function这样的就会率先执行,除这两个之外的就会放在后面执行).这个就解释了上面 第一种和第二种情况下,结果是1的原因:那第一个来说:
第一步:
function eat(){
alert(1);
}第二步:
var eat(即在内存的栈里面申明一个变量名字叫eat)第三步
function(){
alert(2);
}第四步
eat();第五步
将内存里面堆当中的关于
function(){
alert(2);
}
的内存的地址赋值给栈当中的eat变量进行引用所以弹出的结果就是1
接下来说第二种:内存当中,栈是比堆小的,根据上面第一种的情况分析,我们发现了在调用eat()的时候,所有的定义操作全部都执行了,这个时候就要看浏览器的选择了,它就像是一个人一样,也是先从简单的入手,那个方便先来哪个,栈比较小,于是就先从栈当中查找有没有一个变量叫eat,发现有一个,于是栈里面eat的引用的对象就被调用了,于是结果就是2.
扫一扫
96
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
