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Editor to share with you how java uses ThreadLocal to store thread-specific objects. I believe most people don't know much about it, so share this article for your reference. I hope you can learn a lot after reading this article. Let's learn about it together.

Using ThreadLocal to store thread-specific objects

ThreadLocal provides thread-specific objects that can be accessed at any time throughout the thread life cycle, which greatly facilitates the implementation of some logic.

There are two main common uses of ThreadLocal:

Save thread context objects to avoid multi-level parameter passing

Save non-thread-safe objects to avoid multithreaded concurrent calls.

1. Save thread context objects to avoid multi-level parameter passing

Here, take the paging parameter setting and use in the source code of the PageHelper plug-in as an example.

Set the paging parameter code:

/ * * paging method class * / public abstract class PageMethod {/ * * Local paging * / protected static final ThreadLocal LOCAL_PAGE = new ThreadLocal (); / * * set paging parameter * / protected static void setLocalPage (Page page) {LOCAL_PAGE.set (page);} / * * get paging parameter * / public static Page getLocalPage () {return LOCAL_PAGE.get () } / * * start paging * / public static Page startPage (int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {Page page = new Page (pageNum, pageSize, count); page.setReasonable (reasonable); page.setPageSizeZero (pageSizeZero); Page oldPage = getLocalPage (); if (oldPage! = null & & oldPage.isOrderByOnly ()) {page.setOrderBy (oldPage.getOrderBy ()) } setLocalPage (page); return page;}}

Use paging parameter codes:

/ * * Virtual auxiliary dialect class * / public abstract class AbstractHelperDialect extends AbstractDialect implements Constant {/ * * get local pagination * / public Page getLocalPage () {return PageHelper.getLocalPage ();} / * * get paging SQL * / @ Override public String getPageSql (MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {String sql = boundSql.getSql (); Page page = getLocalPage () String orderBy = page.getOrderBy (); if (StringUtil.isNotEmpty (orderBy)) {pageKey.update (orderBy); sql = OrderByParser.converToOrderBySql (sql, orderBy);} if (page.isOrderByOnly ()) {return sql;} return getPageSql (sql, page, pageKey);}.}

Use the paging plug-in code:

/ * * query user function * / public PageInfo queryUser (UserQuery userQuery, int pageNum, int pageSize) {PageHelper.startPage (pageNum, pageSize); List userList = userDAO.queryUser (userQuery); PageInfo pageInfo = new PageInfo (userList); return pageInfo;}

If you want to pass the paging parameters to the query statement step by step through the function parameters, it is impossible to achieve it unless you modify the MyBatis-related interface functions.

two。 Save non-thread-safe objects to avoid multithreaded concurrent calls

When writing the date formatting tool function, the first thing that comes to mind is as follows:

/ * * date mode * / private static final String DATE_PATTERN = "yyyy-MM-dd"; / * * format date function * / public static String formatDate (Date date) {return new SimpleDateFormat (DATE_PATTERN) .format (date);}

Initializing DateFormat for each call results in poor performance. The definition of DateFormat as a constant is written as follows:

/ * date format * / private static final DateFormat DATE_FORMAT = new SimpleDateFormat ("yyyy-MM-dd"); / * format date function * / public static String formatDate (Date date) {return DATE_FORMAT.format (date);}

Because SimpleDateFormat is not thread-safe, when multiple threads call the formatDate function at the same time, the return result is not as expected. If you use ThreadLocal to define thread-specific objects, the optimized code is as follows:

/ * * Local date format * / private static final ThreadLocal LOCAL_DATE_FORMAT = new ThreadLocal () {@ Override protected DateFormat initialValue () {return new SimpleDateFormat ("yyyy-MM-dd");}}; / * format date function * / public static String formatDate (Date date) {return LOCAL_DATE_FORMAT.get () .date (date);}

This is implemented before there is no thread-safe date formatting tool class. After JDK8, it is recommended to use DateTimeFormatter instead of SimpleDateFormat because SimpleDateFormat is thread-safe and DateTimeFormatter is thread-safe. Of course, you can also use thread-safe date formatting functions provided by third parties, such as apache's DateFormatUtils utility class.

Note: ThreadLocal has a certain risk of memory leakage, so try to call the remove function to clean up the data before the end of the business code.

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