本文分析了`java.lang.Object cannot be cast to bjpkten.parsearraydemo.MainActivity$People`错误的原因，指出SparseArray在多线程环境下由于非线程安全可能导致已删除对象的类型转换失败。复现问题的方法是通过创建多个子线程并发地对SparseArray进行读取和删除操作，从而触发该异常。
java.lang.ClassCastException: java.lang.Object cannot be cast to bjpkten.parsearraydemo.MainActivity$People
at bjpkten.parsearraydemo.MainActivity$3.run(MainActivity.java:105)
at java.lang.Thread.run(Thread.java:764)
报错原因：
首先这个报错是因为，
1: SparseArray 删除的时候是假删除，只是将它标记为delete,
2: 在多线程的情况下，看下面的valueAt()这个方法，因为不是线程安全的，所以可能会得到一个已经被删除的数据，
3: 这个时候这个数据因为已经删除了，已经变成了Object 类型了，如果这个时候再执行赋值语句就会导致这个时候报上面的错误。
/**
* Removes the mapping from the specified key, if there was any.
*/
public void delete(int key) {
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
if (i >= 0) {
if (mValues[i] != DELETED) {
mValues[i] = DELETED;
mGarbage = true;
}
}
}
/**
* Given an index in the range <code>0...size()-1</code>, returns
* the value from the <code>index</code>th key-value mapping that this
* SparseArray stores.
*
* <p>The values corresponding to indices in ascending order are guaranteed
* to be associated with keys in ascending order, e.g.,
* <code>valueAt(0)</code> will return the value associated with the
* smallest key and <code>valueAt(size()-1)</code> will return the value
* associated with the largest key.</p>
*
* <p>For indices outside of the range <code>0...size()-1</code>,
* the behavior is undefined.</p>
*/
@SuppressWarnings("unchecked")
public E valueAt(int index) {
if (mGarbage) {
gc();
}
return (E) mValues[index];
}
/**
* Returns the number of key-value mappings that this SparseArray
* currently stores.
*/
public int size() {
if (mGarbage) {
gc();
}
return mSize;
}
private void gc() {
// Log.e("SparseArray", "gc start with " + mSize);
int n = mSize;
int o = 0;
int[] keys = mKeys;
Object[] values = mValues;
for (int i = 0; i < n; i++) {
Object val = values[i];
if (val != DELETED) {
if (i != o) {
keys[o] = keys[i];
values[o] = val;
values[i] = null;
}
o++;
}
}
mGarbage = false;
mSize = o;
// Log.e("SparseArray", "gc end with " + mSize);
}
复现方法：
思路：创建多个子线程，多个子线程同时操作一个SparseArray的对象，对它进行valueAt()的赋值操作和删除操作
package bjpkten.parsearraydemo;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.SparseArray;
import android.view.View;
public class MainActivity extends AppCompatActivity {
private SparseArray<People> sparseArray;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
/**
* 测试子线程使用 sparseArray.valueAt(j);
* @param view
*/
public void tryOnlyValueAt(View view) {
init();
for (int i = 0; i < 10000; i++) {
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int j = 0; j < sparseArray.size(); j++) {
//执行200次的valueAt操作
for (int k = 0; k < 20; k++) {
sparseArray.valueAt(j);
}
}
deleteOneData();
}
}).start();
}
}
/**
* 测试子线程使用 People people1 = sparseArray.valueAt(j); 进行赋值
* @param view
*/
public void tryWithAssignment(View view) {
init();
for (int i = 0; i < 10000; i++) {
new Thread(new Runnable() {
@Override
public void run() {
try {
//这里的sleep 不要太小，不要是1，或者10， 太小一次性就执行完了，不会有多个线程并行执行情况了
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int j = 0; j < sparseArray.size(); j++) {
for (int k = 0; k < 20; k++) {
//执行20次*sparseArray.size 的倍数 valueAt的赋值
People people1 = sparseArray.valueAt(j);
}
}
//随便删除一条数据
deleteOneData();
}
}).start();
}
}
/**
* 测试先判断 if(sparseArray.valueAt(j)!=null && sparseArray.valueAt(j)instanceof People)
* @param view
*/
public void tryWithAssignmentWithJudge(View view) {
init();
for (int i = 0; i < 10000; i++) {
new Thread(new Runnable() {
@Override
public void run() {
try {
//这里的sleep 不要太小，不要是1，或者10， 太小一次性就执行完了，不会有多个线程并行执行情况了
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int j = 0; j < sparseArray.size(); j++) {
for (int k = 0; k < 20; k++) {
//执行20次*sparseArray.size 的倍数 valueAt的判断赋值
if(sparseArray.valueAt(j)!=null && sparseArray.valueAt(j)instanceof People) {
People people1 = sparseArray.valueAt(j);
}
}
}
//顺便删除一条数据
deleteOneData();
}
}).start();
}
}
private void deleteOneData() {
int key = sparseArray.size() -1;
if(key >0) {
//删除一个数据
sparseArray.delete(key );
}
}
/**
* init 初始化 SparseArray
*/
private void init() {
sparseArray = new SparseArray<>();
for (int i = 0; i < 1000; i++) {
sparseArray.put(i,new People(" name " + i));
}
}
class People {
People(String name){
this.name = name;
}
String name;
@Override
public String toString() {
return name + "";
}
}
}
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="每一个子线程都会执行遍历和删除的操作\n
遍历的操作是有所不同，删除都是值删除1个数据"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textColor="#009900"
android:text="点击 测试 10000 次遍历，执行 sparseArray.valueAt(j);"
android:onClick="tryOnlyValueAt"
android:textAllCaps="false"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textColor="#990000"
android:text="（会奔溃概率高）点击 测试10000次 遍历，执行赋值语句 People people = valueAt(j)"
android:onClick="tryWithAssignment"
android:textAllCaps="false"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textColor="#999900"
android:text="（会奔溃但是概率小）点击 测试1000次 遍历，\n 执行判断后再进行赋值 \n
if(valueAt(j)!=null valueAt(j) instanceOf People)\n
People people1 = sparseArray.valueAt(j);"
android:onClick="tryWithAssignmentWithJudge"
android:textAllCaps="false"/>
</LinearLayout>
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