The second command has dropped the " characters from the replacement string. And I don't understand what you are doing with the s at the end, so how about:

sed '/export +JAVA_HOME=/{h;s#=.*#="'$java'"#};$aexport JAVA_HOME="'$java'"' -i ~/.bashrc 

The second command has dropped the " characters from the replacement string. And I don't understand what you are doing with the s at the end, so how about:

sed '/export \{1,\}JAVA_HOME=/{h;s#=.*#="'$java'"#};${x;/^$/{s##export JAVA_HOME="'$java'"#;H};x}' -i ~/.bashrc 

The second command has dropped the " characters from the replacement string.

How about:

sed '/export[ ]JAVA_HOME=/{h;s#=.*#="'$java'"#};${x;/^$/{s##export JAVA_HOME="'$java'"#;H};x}' -i ~/.bashrc 

The second command has dropped the " characters from the replacement string. And I don't understand what you are doing with the s at the end, so how about:

sed '/export +JAVA_HOME=/{h;s#=.*#="'$java'"#};$aexport JAVA_HOME="'$java'"' -i ~/.bashrc