当尝试访问应该存储它的数组列表时，我得到的“tweets”数组的数组大小为零。我正在使用 Jackon 数据绑定(bind)到一个类，此时它已经“工作”了给出任何错误或异常。但是，问题仍然是数组返回空。
这是我访问 JSON 绑定(bind)到的类的方法:
WeatherDatabaseCreator.java
//Read in files to be parsed.
for (int i = 0; i < twitterFiles.size(); i++) {
File twitterFile = twitterFiles.get(i);
WeatherTweetParser wtp = new WeatherTweetParser(twitterFile);
wtp.mapJavaObject();
tws = new Tweets();
tweets = tws.getTweets();
System.out.print(tweets.size());
tweetIndex = 0;
System.out.println("Parsing and inserting twitter file '" + twitterFile.getAbsolutePath() + "'...");
//Step through all the tweets in each file, using the TweetParser class.
while (true) {
tweet = tweets.get(tweetIndex);
user = tweet.new User();
entity = tweet.new Entities();
userMentions = entity.getUserMentions();
hashtags = entity.getHashtags();
if (tweet.getText() == null) {
break;
}
numParsed++;
Boolean success = wdbm.insertTweet(tweet, user);
if (success == true) {
numInserted++;
}
if (entity.getNumHashtags() > 0){
for (int j = 0; j < entity.getNumHashtags(); j++) {
wdbm.insertHashtag(tweet, hashtags.get(j));
}
}
if (entity.getNumUserMentions() > 0) {
for (int k = 0; k < entity.getNumUserMentions(); k++) {
wdbm.insertReference(tweet, userMentions.get(k));
}
}
if (numParsed % UPDATE_INTERVAL == 0) {
System.out.println(numParsed + " tweets parsed. " + numInserted + " tweets inserted into database...");
}
tweetIndex++;
}
这是 JSON 文件的一部分。该文件由一组推文和一些嵌套类组成。这里我包含了数组的两个元素:
2014-01-03-11-03.terms.json
{
"tweets": [
{
"filter_level": "medium",
"contributors": null,
"text": "U know your ice fishing at home when ur snacks are sandwiches,chips,dips,beef jerky and all the goodies instead of just a case of beer #mom",
"geo": null,
"retweeted": false,
"in_reply_to_screen_name": null,
"truncated": false,
"lang": "en",
"entities": {
"symbols": [
],
"urls": [
],
"hashtags": [
{
"text": "mom",
"indices": [
135,
139
]
}
],
"user_mentions": [
]
},
"in_reply_to_status_id_str": null,
"id": 419137025376145408,
"source": "<a href=\"http://twitter.com/download/iphone\" rel=\"nofollow\">Twitter for iPhone<\/a>",
"in_reply_to_user_id_str": null,
"favorited": false,
"in_reply_to_status_id": null,
"retweet_count": 0,
"created_at": "Fri Jan 03 16:03:48 +0000 2014",
"in_reply_to_user_id": null,
"favorite_count": 0,
"id_str": "419137025376145408",
"place": null,
"user": {
"location": "",
"default_profile": false,
"profile_background_tile": true,
"statuses_count": 5411,
"lang": "en",
"profile_link_color": "17EBCF",
"profile_banner_url": "https://pbs.twimg.com/profile_banners/408240224/1384838946",
"id": 408240224,
"following": null,
"protected": false,
"favourites_count": 4222,
"profile_text_color": "333333",
"description": "It is what it is, no more, no less. Trust in the Lord. BSU Softball #5. My whole being is happy - Psalm 16:9",
"verified": false,
"contributors_enabled": false,
"profile_sidebar_border_color": "FFFFFF",
"name": "Nicole Anne",
"profile_background_color": "C0DEED",
"created_at": "Wed Nov 09 05:03:58 +0000 2011",
"default_profile_image": false,
"followers_count": 452,
"profile_image_url_https": "https://pbs.twimg.com/profile_images/413270299925164033/mWiJBKP2_normal.jpeg",
"geo_enabled": true,
"profile_background_image_url": "http://a0.twimg.com/profile_background_images/698793822/14d9ce538d96b435ad4449d5385a3ec1.jpeg",
"profile_background_image_url_https": "https://si0.twimg.com/profile_background_images/698793822/14d9ce538d96b435ad4449d5385a3ec1.jpeg",
"follow_request_sent": null,
"url": null,
"utc_offset": -32400,
"time_zone": "Alaska",
"notifications": null,
"profile_use_background_image": true,
"friends_count": 634,
"profile_sidebar_fill_color": "DDEEF6",
"screen_name": "NDupay",
"id_str": "408240224",
"profile_image_url": "http://pbs.twimg.com/profile_images/413270299925164033/mWiJBKP2_normal.jpeg",
"listed_count": 1,
"is_translator": false
},
"coordinates": null
},
{
"filter_level": "medium",
"contributors": null,
"text": "Just watched DeBlasio shovel snow for the cameras and I have to say, his form is terrible. Looks a bit inexperienced.",
"geo": null,
"retweeted": false,
"in_reply_to_screen_name": null,
"truncated": false,
"lang": "en",
"entities": {
"symbols": [
],
"urls": [
],
"hashtags": [
],
"user_mentions": [
]
},
"in_reply_to_status_id_str": null,
"id": 419137026458673152,
"source": "<a href=\"http://twitter.com/download/iphone\" rel=\"nofollow\">Twitter for iPhone<\/a>",
"in_reply_to_user_id_str": null,
"favorited": false,
"in_reply_to_status_id": null,
"retweet_count": 0,
"created_at": "Fri Jan 03 16:03:48 +0000 2014",
"in_reply_to_user_id": null,
"favorite_count": 0,
"id_str": "419137026458673152",
"place": null,
"user": {
"location": "",
"default_profile": false,
"profile_background_tile": false,
"statuses_count": 2609,
"lang": "en",
"profile_link_color": "FF8000",
"id": 163686045,
"following": null,
"protected": false,
"favourites_count": 204,
"profile_text_color": "333333",
"description": "Love learning, Hockey #NYR, music and photography. My family is my whole life.",
"verified": false,
"contributors_enabled": false,
"profile_sidebar_border_color": "EEEEEE",
"name": "Steven Marques",
"profile_background_color": "ACDED6",
"created_at": "Wed Jul 07 01:26:25 +0000 2010",
"default_profile_image": false,
"followers_count": 56,
"profile_image_url_https": "https://pbs.twimg.com/profile_images/378800000795958214/5e3ff928282ad692a36e941e5c5ff628_normal.jpeg",
"geo_enabled": false,
"profile_background_image_url": "http://abs.twimg.com/images/themes/theme18/bg.gif",
"profile_background_image_url_https": "https://abs.twimg.com/images/themes/theme18/bg.gif",
"follow_request_sent": null,
"url": null,
"utc_offset": -18000,
"time_zone": "Eastern Time (US & Canada)",
"notifications": null,
"profile_use_background_image": true,
"friends_count": 58,
"profile_sidebar_fill_color": "F6F6F6",
"screen_name": "SMGafanha",
"id_str": "163686045",
"profile_image_url": "http://pbs.twimg.com/profile_images/378800000795958214/5e3ff928282ad692a36e941e5c5ff628_normal.jpeg",
"listed_count": 2,
"is_translator": false
},
"coordinates": null
},
这里是 JSON 文件映射到的 Java 类，后面是将 JSON 映射到类的类:
Tweets.java
public class Tweets {
private ArrayList<Tweet> tweets;
public Tweets() {
tweets = new ArrayList<Tweet>();
}
public ArrayList<Tweet> getTweets() {
return tweets;
}
public void setTweets(ArrayList<Tweet> tweets) {
this.tweets = tweets;
}
public static class Tweet {
private String text;
@JsonProperty("source")
private String postMethod;
@JsonProperty("created_at")
private String time;
@JsonProperty("id")
private String ID;
public Tweet() {
}
public String getText() {
return text;
}
public void setText(String text) {
this.text = text;
}
public String getID() {
return ID;
}
public void setID(String iD) {
ID = iD;
}
public String getPostMethod() {
return postMethod;
}
public void setPostMethod(String postMethod) {
this.postMethod = postMethod;
}
public String getTime() {
return time;
}
public void setTime(String time) {
this.time = time;
}
public class User {
private String location;
@JsonProperty("screen_name")
private String user;
@JsonProperty("time_zone")
private String timeZone;
@JsonProperty("description")
private String accountDescription;
public User(){
}
public String getLocation() {
return location;
}
public void setLocation(String location) {
this.location = location;
}
public String getUser() {
return user;
}
public void setUser(String user) {
this.user = user;
}
public String getTimeZone() {
return timeZone;
}
public void setTimeZone(String timeZone) {
this.timeZone = timeZone;
}
public String getAccountDescription() {
return accountDescription;
}
public void setAccountDescription(String accountDescription) {
this.accountDescription = accountDescription;
}
}
public class Entities {
@JsonProperty("user_mentions")
ArrayList<UserMention> userMentions;
@JsonProperty("hashtags")
ArrayList<Hashtag> hashtags;
public Entities() {
}
public ArrayList<UserMention> getUserMentions() {
return userMentions;
}
public int getNumUserMentions() {
return userMentions.size();
}
public UserMention getUserMention(int index) {
return userMentions.get(index);
}
public void setUserMentions(ArrayList<UserMention> userMentions) {
this.userMentions = userMentions;
}
public ArrayList<Hashtag> getHashtags() {
return hashtags;
}
public int getNumHashtags() {
return hashtags.size();
}
public Hashtag getHashtag(int index) {
return hashtags.get(index);
}
public void setHashtags(ArrayList<Hashtag> hashtags) {
this.hashtags = hashtags;
}
public class UserMention {
@JsonProperty("screen_name")
private String userRef;
public UserMention() {
}
public String getUserRef() {
return userRef;
}
public void setUserRef(String userRef) {
this.userRef = userRef;
}
}
public class Hashtag {
@JsonProperty("text")
private String hashText;
public Hashtag() {
}
public String getHashText() {
return hashText;
}
public void setHashText(String hashText) {
this.hashText = hashText;
}
}
}
}
WeatherTweetParser.java
public class WeatherTweetParser {
private static File twitterFile;
public WeatherTweetParser(File twitterFile) {
WeatherTweetParser.twitterFile = twitterFile;
}
public void mapJavaObject() {
String jsonFile = twitterFile.getName();
System.out.println("Mapping JSON to WeatherTweet Class.");
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
try {
mapper.readValue(new File(jsonFile), Tweets.class);
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
System.out.println("Finished Mapping of JSON File.");
}
这里是错误:
Mapping JSON to WeatherTweet Class.
Finished Mapping of JSON File.
Tweets size: 0
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.get(Unknown Source)
at database.WeatherDatabaseCreator.main(WeatherDatabaseCreator.java:145)
Parsing and inserting twitter file 'C:\Users\Jared\Documents\Eclipse
WorkSpace\workspace\WeatherDatabaseCreator\2014-01-03-11-03.terms.json'...
在 WeatherDatabaseCreator.java 的这一行:
tweet = tweets.get(tweetIndex);
TL:DR:我理解这个错误是因为索引超出范围，因为即使在 tweets 数组中也没有明显的内容。但是，如果 JSON 文件到我的 Java 类的 Jackson 映射无一异常(exception)地通过了，为什么 tweets 数组是空的？
最佳答案
while (true) {
tweet = tweets.get(tweetIndex);
user = tweet.new User();
entity = tweet.new Entities();
userMentions = entity.getUserMentions();
hashtags = entity.getHashtags();
if (tweet.getText() == null) {
break;
}
只有当 getText 为 null 时，此循环中的唯一中断才会起作用。如果所有推文的 getText 都不为空，您将得到索引超出范围的异常。 而不是 while(true) 你应该使用像 tweetIndex < tweets.size()
这样的检查第二个问题。您创建了一个 Tweets 对象，但没有在其中加载任何项目:
wtp.mapJavaObject();
tws = new Tweets();
tweets = tws.getTweets();
当然，在这种情况下，tws 对象不会有任何推文。您需要做的是确保您的 mapJavaObjects() 方法返回一个包含推文的 Tweets 对象由 ObjectMapper 加载，完成后您的代码应如下所示:
tweets = wtp.mapJavaObject();
像这样:
public Tweets mapJavaObject() {
String jsonFile = twitterFile.getName();
Tweets ret = null;
System.out.println("Mapping JSON to WeatherTweet Class.");
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
try {
ret = (Tweets) mapper.readValue(new File(jsonFile), Tweets.class);
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
System.out.println("Finished Mapping of JSON File.");
retrun ret;
}
关于java - Jackson ObjectMapper 数组中的 JSON 空指针异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23834744/