Java:如果访问是只读的，两个线程是否可以安全地访问同一个方法

前言

我一天中只编写了几个多线程程序.当我必须这样做时，通常每两年一次.我正在尝试获得更多关于该主题的教育，我正在阅读实践中的 Java 并发".我有一个基本的了解.

I have only written a couple of multithreaded programs in my day. It's usually once every two years when I have to do this. I'm trying to get more educated on the subject and I'm reading "Java Concurrency in Practice." I have a basic understanding.

概述:

通常，我从不跨线程共享对象，因为它更容易，而且在大多数情况下，我只是想避免基本的阻塞情况.但是，我有一个用例，其中跨线程共享对象是有意义的.

Typically, I never share objects across threads because it's easier and in most cases I'm just trying to avoid basic blocking scenarios. However, I have a use case where it makes sense to share an object across threads.

我的 JSONBmBindMappingRow 在我的主线程中实例化(此处不包括不同的类).私有对象 BMBindMappingRow 在 JSONBmBindMappingRow 中设置.您可以将 JSONBmBindMappingRow 类视为不可变的；虽然，它绝对不是.但是，我将在我的程序中以这种方式对待它.

My JSONBmBindMappingRow is instantiated in my main thread (different class not included here). A private object BMBindMappingRow is set in JSONBmBindMappingRow . You can think of the JSONBmBindMappingRow class as immutable; although, it's definitely not. However, I will be treating it that way in my program.

JSONBmBindMappingRow 实例化后，它可以分配给多个线程，这些线程将调用 getJsonRow().

After the JSONBmBindMappingRow is instantiated it can be assigned to multiple threads which will call getJsonRow().

问题:

我的问题的范围如下:如果两个或多个线程同时访问 getJsonRow() 是否该线程安全，因为它们在自己的内存缓存中都有 JSONBmBindMappingRow 的副本?我认为它是安全的，不需要同步，但我会把它留给专家.

The scope of my question is as follows: If two or more threads access the getJsonRow() at the sametime is this thread-safe since both will have a copy of the JSONBmBindMappingRow in there own memory cache? I think it's safe and synchronization is not needed, but I will leave it to the experts.

如果两个线程同时访问，这段代码线程安全吗?

  public JSONRow getJsonRow()
  {
    JSONRow jrow = new JSONRow();
    for (Integer index: bbmr.getColumnMappingAll().keySet()) {
       BMFieldMapping bm = bbmr.getColumnMapping(index);
       if (bm.ws_field_name != null && !bm.ws_field_name.equalsIgnoreCase("")) {
          jrow.add(new JSONField(bm.ws_field_name, bm.getJavaDataType(), 1));
       }
    }
    return jrow;
  }

JSONBmBindMappingRow 类:

  package xxfi.oracle.apps.ws.json.row;

  import java.sql.Connection;
  import java.sql.SQLException;

  import oracle.jdbc.OracleCallableStatement;
  import oracle.jdbc.OracleResultSet;
  import oracle.jdbc.OracleTypes;

  import xxfi.oracle.apps.ws.bm.BMBindMappingRow;
  import xxfi.oracle.apps.ws.bm.BMFieldMapping;
  import xxfi.oracle.apps.ws.utility.JDBC;


  public class JSONBmBindMappingRow implements JSONRowBuildImpl
  {
     private BMBindMappingRow bbmr = new BMBindMappingRow();
     private Connection conn = null;
     private String tableName = null;
     private String className = this.getClass().getCanonicalName() ;

     public JSONBmBindMappingRow(Connection conn, String tableName)
     {
        this.tableName = tableName;
        this.conn = conn;
        init();
     }

     public void init()
     {
        setColumnBindMappings();
     }


     public void setColumnBindMappings()
     {
        StringBuffer plSql = new StringBuffer();

        plSql.append("DECLARE ");
        plSql.append("BEGIN ");
        plSql.append("   :1 := xxfi_bm_custom_table_api.get_column_binds ( ");
        plSql.append("  :2       /*tablename*/");
        plSql.append(");");
        plSql.append("END;");

        OracleCallableStatement oracleCallableStatement = null;
        OracleResultSet oracleResultSet = null;
        try {
           oracleCallableStatement = (OracleCallableStatement) this.conn.prepareCall(plSql.toString());
           oracleCallableStatement.registerOutParameter(1, OracleTypes.CURSOR);
           JDBC.nullSafe(oracleCallableStatement, 2, tableName, OracleTypes.VARCHAR);

           oracleCallableStatement.execute();

           // get cursor and cast it to ResultSet
           oracleResultSet = (OracleResultSet) oracleCallableStatement.getCursor(1);

           // loop it like normal
           while (oracleResultSet.next()) {
              bbmr.add(new BMFieldMapping(oracleResultSet.getString("ws_field_name"), 
                       oracleResultSet.getString("column_name"), oracleResultSet.getString("data_type"), 
                       oracleResultSet.getInt("bind_number")));

           }
         } catch (Exception e) {
           try {
              this.conn.rollback();
           } catch (SQLException f) {
              // TODO
           }
           System.out.println("Error in "+className+".setColumnBindMappings(): " + e);
           e.printStackTrace();

        } finally {
           JDBC.close(oracleCallableStatement, oracleResultSet);
        }
     }

     public String getArrayName()
     {
        return "";
     }

     public JSONRow getJsonRow()
     {
        JSONRow jrow = new JSONRow();
        for (Integer index: bbmr.getColumnMappingAll().keySet()) {
           BMFieldMapping bm = bbmr.getColumnMapping(index);
           if (bm.ws_field_name != null && !bm.ws_field_name.equalsIgnoreCase("")) {
              jrow.add(new JSONField(bm.ws_field_name, bm.getJavaDataType(), 1));
           }
        }
        return jrow;
     }

     public BMBindMappingRow getBbmr()
     {
        return bbmr;
     }
  }